Open up the Ships/Units icon. Your total units in space is reported there. The absolute maximum effective for settings.txt is 5000 each, verified. I watch that number very closely and compare against what still needs to be launched. You can try bringing fighters/ sats back into your colony cargo bays to make more room. Mines once deployed cannot be recovered. BUT if you no longer need a stack of mines, you can select it then go into the scrap icon and self destruct the entire stack.