You can see the number of licenses (and level of access) that you have on your billing page. Keep in mind that this includes a free trial and a free-level tier.
When you don't have enough licenses for your account, we will disable access for certain users. Your licenses are allocated to the earliest-added, active users. If a user does not have access, they will see an error when they log in. You will see "License Missing" on the account users page.
To fix this, visit your billing page and increase the quantity of licenses to match the number of active users on your account.