How many taps should a font have?

As many lines as you expect to run within a year or two, not just what you are pouring today. The counter is cut to suit the font, so adding a line later is a much bigger job than specifying for it now.

Can I change the badge on a font?

If it uses standard badge holders, yes, and that is worth insisting on. Bars that rotate guest kegs change badges constantly and a font that locks you to one brand becomes a nuisance.