I'm not sure but I found him on floor 62.
Answer (by skyra) (i dont know how to make a new answer thing)
You can find Mew in any floor after floor 45 (After you beat Registeel). You just need to have a music box and the Final Island friend area.
P.S. To get Mewtwo you need the Cryptic Cave friend area- you can find both these friend areas at the friend area shop.
AnswerMew is not found on specific floors, but randomly on a range of floors from B36F to B98F. You must have defeated the three Regi Pokemon on your way through the dungeon (or simply pass their floors if you have recruited them) and you must have in your possession the Music Box. This item is created from the Rock, Ice and Steel parts which are found after you have defeated the three Regi Pokemon.
The Music Box is crucial to you finding Mew. Without it, it will not appear on any floor. Also keep in mind that if you do encounter Mew, it will most likely be a Sandshrew or any other Pokemon found on that level as Mew uses Transform. It will still show on the message screen as 'Mew' and it will still have it's ability to hover over water tiles.
If you defeat Mew once in the Buried Relic, do not think that it is gone forever. You may decend to further levels and find it again. It will be random, but you can find Mew more than once while you are in the Buried Relic. Take note that if you wish to recruit Mew, you will need space for it in your party and you also need patience as it is very difficult to obtain - even if your lead Pokemon is above level 90 with a Freind Bow. (Just remember that you need the Friend Area Final Island to recruit Mew, but not to encounter it.)
In essence, Mew is found on floors 46, 68 and 98 of the Buried Relic, but it can be found on all the floors in between those. It is merely up to chance.
36f - 98f :D + you need a music box ;) x
OK I agree with the above answers, from 36F-98F......and i advise you to take LOADS AND LOADS of normal, big and huge apples because it is a heck of a long journey. I took about 7 or 8 and i still nearly fainted, i only survived because my Eevee knows rest and because I had some reviver seeds. I would take lots of max exlirs and reviver seeds as well, just to be safe. Hope this helps ;)
The dungeon Buried Relic consists of 99 floors.
99
99
48 floors
A dungeon is a cave on pokemon mystery dungeon. It is a level with a series of floors/rooms and the object is to get to the end of the dungeon.
In buried relic on floors 15 25 and 35
30 floors
there is thirty floors
24 floors
there are 20 floors.
40
The Nightmare has 17 floors.