You can in most cases put 20 breakers in a 100 amp panel.
However some 100 amp breaker panels can hold up to 42 breakers but it will depend on the breaker panel design.
Standard 100 amp breaker panels hold 20 circuit breakers but some can hold more.
However the total amperage of all the breakers combined in the breaker panel should however never exceed the 100 amp breaker panels rated amperage.
However some breaker panels may fit more breakers in the breaker panel that may exceed the breaker panels amperage rating the 100 amp main breaker that feeds the panel should trip if you exceed 100 amps.
If your breaker panel doesn't have a main 100 amp breaker feeding it then the panel could overheat and not trip when you exceed 100 amps.
In most cases you won't be pulling the entire amperage of the breaker panel or 100 amp breaker at one time and most times it will be okay.
But a breaker panel should have a main breaker when possible to protect the breaker panel and your home should you exceed that amperage.