How many amps are in the Roland AC range?

The Roland AC range is comprised of the following: AC-90, AC-60, AC-40, AC-33, Mobile AC.

What is that connection in the mic channel?

That is an XLR input, which is a very low noise connection for microphones. Using an XLR cables between your mic and your amp allows a high signal level to come through with a low noise floor.

Which amp is best for paying small gigs with?

We suppose it depend so whether there is PA availabel at the venue or not. We'd always say that having more power means more volume if you need it and more headroom for a clean sound at high volumes so we'll recommend the AC-90.