There's a type of research called "fleet supply" that increases the number of units you can build. However, it also reduces your income (permanently!) when you research it, so be careful not to do it too early or too often. The first upgrade is a simple 9% tax on all income, but when the last upgrade increases it to 75% of your income, leaving you with only a fraction of your gross income.
Have fun, and if you need any help or advice, feel free to ask!
