Depending on the account type you have chosen, and the number of paid licenses/user in that account, there is total number of new events that can be added each month. This is a pool and they can be added by any user. At the end of each month your usage is reset.
For example with a Business account you can add a total 1000 new events if you just have 1 license. If you have 5 paid licenses in your Business account, you can add up to 5000 events each month. Adding licenses to will increase the allowance on your account.