How many silent consonants are in tonight?
Thousandths. Strengths. Lengths. Twelfths. Also, there are many more if you include compound words : "Latchstring" has six consonants, as does "catchphrase" (a 20th century neologism). There are many proper names included as well, such as Knightsbridge (an area of London, though the "gh" is silent).
If you count Y as a consonant, there are 21 consonants. If you count Y as a vowel, there are 20 consonants. The preceding answer deals only with the ways of WRITING consonants using the Latin alphabet. There are, in fact, around 25 distinctive consonants (sounds made by blocking the air as it moves through the mouth) in the English language.