Each creature you control with a level counter on it gets just +2/+2, regardless of how many level counters it has. It doesn’t get the bonus for each counter.

Time of Heroes affects any creature you control with a level counter on it, not just levelers. (Of course, it’s pretty hard to get level counters onto a creature that’s not a leveler, but it can be done with copy effects, Fate Transfer, or Quicksilver Elemental, among other possibilities.)