It depends which dictionary you use, but roughly 44:
airn, akin, alif, anil, aril, fail, fain, fair, farl, fiar, fila, fink, firn, flak, flan, flir, ilka, kaif, kail, kain, karn, kiln, kina, kirn, knar, lain, lair, lank, lari, lark, liar, link, linn, lira, naif, nail, nark, rail, rain, raki, rani, rank, rial, rink.
Words that can be made from the letters in Benjamin are:
3-letter words
aal, aba, ail, aim, ain, air, ala, alb, ale, ama, ami, ana, ane, ani, arb, are, arf, ark, arm, baa, bal, bam, ban, bar, bel, ben, bin, bra, ear, elf, elk, elm, era, ern, fan, far, fem, fen, fer, fib, fie, fil, fin, fir, ilk, ink, inn, ire, irk, jab, jam, jar, jib, jin, kab, kae, kaf, kea, kef, ken, kif, kin, kir, lab, lam, lar, lea, lei, lek, lib, lie, lin, mae, man, mar, mel, men, mib, mil, mir, nab, nae, nam, neb, nib, nil, nim, raj, ram, ran, reb, ref, rei, rem, ria, rib, rif, rim, rin
4-letter words
able, abri, afar, airn, ajar, akin, alae, alan, alar, alba, alef, alfa, alif, alma, alme, amen, amia, amie, amin, amir, anal, anil, anna, arak, area, aria, aril, baal, bail, bake, bale, balk, balm, bane, bani, bank, bare, barf, bark, barm, barn, beak, beam, bean, bear, bema, berm, bier, bike, bile, bilk, bima, bine, birk, birl, blae, blam, blin, brae, bran, bren, brie, brim, brin, earl, earn, elan, emir, fail, fain, fair, fake, fame, fane, fare, farl, farm, feal, fear, fern, fiar, fila, file, film, fine, fink, fire, firm, firn, flab, flak, flam, flan, flea, frae, iamb, ilea, ilia, ilka, inia, jail, jake, jamb, jane, jarl, jean, jerk, jibe, jink, jinn, kaif, kail, kain, kale, kame, kami, kana, kane, karn, kbar, keir, kerb, kerf, kern, kibe, kief, kier, kiln, kina, kine, kirn, knar, lain, lair, lake, lama, lamb, lame, lane, lank, lari, lark, leaf, leak, lean, lear, liar, lief, lien, lier, life, like, lima, limb, lime, limn, line, link, linn, lira, lire, maar, mabe, mail, main, mair, make, male, mana, mane, mare, mark, marl, meal, mean, merk, merl, mien, mike, mile, milk, mina, mine, mini, mink, mire, miri, mirk, nabe, naif, nail, name, nana, nark, near, neif, nema, nine, raia, rail, rain, raja, rake, raki, rale, rami, rani, rank, real, ream, reif, rein, rial, riel, rife, rile, rime, rink
5-letter words
aalii, abeam, abler, afire, aimer, akela, alane, alarm, alibi, alien, alike, aline, amain, amber, amble, ameba, amine, amnia, anear, anile, anima, anime, animi, ankle, annal, areal, arena, ariel, bairn, baker, baler, banal, benni, biali, biker, bikie, binal, birle, blain, blame, blank, blare, bleak, blear, blini, blink, brail, brain, brake, brank, break, bream, brief, brine, brink, elain, elfin, embar, fable, faena, faker, fakir, farle, feral, feria, fermi, fiber, fibre, filar, filer, final, finer, flair, flake, flame, flank, flare, fleam, flier, frail, frame, frank, freak, frena, iambi, imine, inane, inarm, infer, infra, inker, inkle, inner, jambe, jiber, jinni, jnana, kabar, kafir, kalam, kalif, kanji, karma, kebar, kefir, kenaf, kibei, kibla, kilim, kinin, knife, kraal, labia, labra, laker, lamer, lamia, lanai, learn, leman, liana, liane, liber, libra, libri, lifer, liken, liker, liman, limba, limbi, limen, linen, liner, linin, mafia, maile, makar, maker, malar, mania, manna, maria, mbira, mikra, miler, milia, minae, miner, minke, naira, namer, nimbi, ninja, ramie, realm, reink, reman, renal, renin, rifle
6-letter words
aerial, aflame, airman, airmen, alanin, alfaki, aliner, alkane, alkine, almner, ambari, ambler, ameban, ananke, anemia, anilin, animal, anneal, arable, baalim, bailer, bailie, balker, banian, banker, banner, barman, barmen, barmie, berlin, bilker, birkie, blamer, embank, embark, fabler, fainer, famine, fanner, farina, ferbam, ferial, fibril, fibrin, filmer, fimble, finale, finial, firkin, firman, flaker, flambe, flamen, flamer, fraena, imbark, inaner, infare, infirm, inkier, inlier, jailer, jarina, jerkin, jinker, kalian, kalmia, kinema, knifer, lakier, lamber, lambie, lamiae, lamina, lanker, lanner, larine, librae, limber, limier, limina, limner, linear, linier, linker, mailer, maline, malkin, manila, mannan, manner, marble, marina, marine, marlin, menial, merlin, milker, milneb, nailer, nankin, nannie, narial, narine, nimble, ramble, rankle, realia, refilm, relink, remail, remain, renail, rennin
7-letter words
abelian, airlike, airline, airmail, alanine, amiable, aniline, armlike, balkier, balmier, biennia, bifilar, blanker, blinker, fanlike, filaria, filmier, fimbria, finable, finlike, finmark, finnier, fireman, flakier, flamier, flanken, flanker, friable, ikebana, inflame, kalimba, kremlin, lambkin, laminae, laminar, lankier, limbier, lineman, linkman, linkmen, makable, manakin, manikin, manlier, manlike, marline, melanin, milkier, minable, mineral, namable, niblike, nimbler, ramekin, ramilie, riblike
8-letter words
barnlike, benjamin, biennial, bilinear, brakeman, familiar, farmable, fibranne, filariae, filarian, fimbriae, fimbrial, finnmark, framable, franklin, inarable, infernal, inflamer, mainline, mannikin, melanian, ninebark, renminbi, rifleman
9-letter words
animalier, frankable, jailbreak, lamebrain, laminarin
Five letter words that can be made with the letters in 'Thanksgiving' are:agingangstankhsgainsgaitsgangsgiantgnashgnatshangshankshintskingsknishknitsnightninthsaintsatinshanksightstainstankstingstinktanksthankthingthinkthinsvisitvista
Words that can be made from the letter is 'with' are:hihitIitwit
Words that can be made with the letters 'aer' are:aareearera
Four letter words that can be made from the letters in 'twinkle' are:kilnkiltkiteknewknitlentlienlikelinelinklintnewttiletinetwinweltwentwilewiltwinewink
Five letter words that can be made from the letters in 'knighthood' are:dingodoingingotnightthighthingthinkthong
Four letter words that can be made from the letters in 'Olympics' are:clipclopcloycoilcopylimplispmoiloilyployposysiloslimslipslopsoil
Three letter words:aidanddindipdunnapnippadpanpinpun
beefsbeefybeetsbesetbestsbytescystsfecesmeetsmessymetessectsseemsstemsstyesteemsyeseszebeczestszestyzymes
Words that can be made from the letter in meal are:aalealmamlamlamemaleme
Five letter words that can be made from the letters 'one hundred' are:droneendedendererodeheronhonedhordehoundroundudderunder
The two letter words that can be made from the word 'cooperate' are:atorpato
Three letter words that can be made with the letters 'bingo' are:bigbinboggingobionnibobi