In order to spawn more Bricktrons, you need to gather blue crystals. The only current way to see how many crystals you have, is to click the big Crystal you spawn beside and check it's spawn %.
As of version 9.0, you can only spawn 1 type of Bricktron. You can, however, equep the Bricktrons with bow and arrow or sword and shield. For this you will need Iron, a Furnace, a Workbench, and a Weapon Rack. Que up iron ingots at the Furnace and then que up weapons at the Workbench. The Bricktons will automatically craft the desired items.
Spawning for old verion (< 9.0):