99.999999999999999 (to infinity with decimal)
4
19 numbers.
91,97 are the two large prime numbers below 100.
There are two perfect numbers, 6 and 28, that are less than 100.
25
1060
4 of them.
'from 100' ??? If you mean 'less than 100' then there are 25 primes. If you mean 'more than 100' then the answer is infinite, just like numbers are infinite.
The 3 digit numbers under 500 are 100 through 499.
the largest prime number under 100 is 97
The highest prime number under 100 is 97.
It's not a case of working them out, you just have to know them. Here are the following prime numbers under 100: