You are limited in how many units you can have at any time. Each unit type has a size rating. Your units' size ratings are added together to determine how much of your capacity they'll take up. Once you have reached capacity, you will not be able to build new units without first scrapping some.
When you click on your Storage building, you will see 2 numbers under the header "Storage Capacity"...
The first number is how much capacity is already taken up by the units in your base. The second number is the maximum total capacity that is available for the current level of your Storage building.
Alternatively, when you view the Storage building popup, you will see the maximum capacity again as well as the listed capacity of all units both in your base and deployed on the world map.