"How much kata is in Kenpo (or Kempo) compared with Shotokan?
or Kyukushin?"
Generally there are fewer kata in Kenpo (Parker lineage), about seven or so empty-hand kata. Shotokan has at least 25, often more; Kyokushin has over a dozen.
"Do kempo artists do katas even in higher belts?"
A martial artist who practices unarmed forms continues to study their forms, always. While I'm unsure if any new kata are added (I believe weapons kata are added, but that might be school-specific), when kata is used as a learning tool, it's constantly being reviewed.
"How much time will be needed for it to effective in street self defense when trained regularly (2x a week)"
As long as it takes. There's no formula for this. It depends on how the system is trained, how good the teacher is, how good the student is, what nature of attack they're facing, what mindset they're in, what mindset their attacker is in, etc. The good news is, the punch or block you learn in your first class will usually work as advertised under the right circumstances. The bad news is, you can't pick how you're going to be attacked.