Does exchange that supports a lot of coins are better than exchange that supports a few coins? (50 - 70 coins vs 5 - 7)
How do u think?

I think a larger variety of coins is better, more markets the better. But it depends on whether they can handle that many wallets.

why not?

Wallets are memory-intensive, so they would have to have a good infrastructure in order to not break under pressure.