You can increase your base's population by adding and upgrading Housing buildings. Some other buildings will also provide extra population, like Janine's Farmhouse. You can add as many Housing buildings as you like and aren't restricted to just one housing block.
You can check the size of your population by looking at the number in at the top of the base builder screen - it looks like 25/90. This means that your base currently has a population of 25 people, and it has a population cap of 90.
To increase your population cap, you need to add and upgrade buildings like Farms and the Hospital.