Buttondown is free for your first hundred subscribers and requires a small monthly fee for more than that, because sending emails and running servers ain't cheap! (You can read more on our pricing page.)
That being said, it would be awful if you suddenly hit a hundred subscribers and didn't have a chance to register a credit card and so additional subscribers would be shown the door — that doesn't happen! You can continue to accrue subscribers past a hundred, you just can't send any emails to them until you fill out your billing information.