Learn French: Il y a trente-cinq clients dans le restaurant. - There are 35 clients in the restaurant.

In French, compound numbers smaller than 100 are always connected with a hyphen. So, 30 (trente) and 5 (cinq) join together to become trente-cinq.

Does il y a mean "there is" or "there are" here?

It means both! Unlike English, which switches between "there is" for one thing and "there are" for multiple things, French uses il y a for both. Since it is followed by 35 clients, it translates to "there are" in this sentence.

Do I pronounce the 's' at the end of clients?

No, the final 's' added to make nouns plural is almost always silent in French. You also do not pronounce the 't' before it. The number trente-cinq already makes it perfectly clear that we are talking about more than one client!

Why do we say dans le restaurant instead of just dans restaurant?

The word dans only means "in" or "inside." French almost always requires an article (like "the" or "a") before a noun. Because restaurant is a masculine word, we use le along with dans to say "in the restaurant."