How Do I Know How Many Seats Are in My Team?

You can check the number of all paid seats in your Team and how many of them are used in the Team Admin space next to the name of your Team: 
In the example below you can see that altogether there are 70 seats in the Team, but there are 25 Team members. It means, that a Team Owner can invite 45 more persons to the Team. 

Swivl made it easy for me to handle my issue.