Learn French: Vous avez tort, Monsieur.

Just like with age and physical states (avoir faim, avoir soif), French uses the verb avoir (to have) for being wrong. The expression is avoir tort. Literally, this means "You have wrong" or "You have fault". Never say Vous êtes tort!

Is the final 't' in tort pronounced?

No, the final t in tort is silent, so it sounds like "tor". In French, consonants at the very end of a word are usually not pronounced unless they are C, R, F, or L.

Does tort change its spelling if I say this to a woman or a group?

No, tort never changes. Even though English uses "wrong" as an adjective, in French tort is a noun meaning "fault". Because it's part of a set expression with avoir, it doesn't get an -e for feminine or an -s for plural.

How can we tell if Vous is singular or plural in this sentence?

The word Monsieur (Sir) tells us the speaker is talking to just one person. Here, vous is used as the formal, polite "you" for a single adult. If it were a group of men, the sentence would end with Messieurs.