How many months have 28 days if some months have 31 days and some have 30 days?

They all have 28 days. Some have more than that, February has exactly that, but there is always a 28th in every month.

The only exception to that would be October of 1582 when the Pope declared that October 5th - 14th would be skipped (because the calendar was off due to no leap year up ot that point).