Stealth Drones are for scouting out enemy bases only. It's recommended that you scout potential targets first. Build as many Stealth Drones (900-1200 units) as your base can launch to ensure a successful scout and destroy any defending scouts at the enemy base with minimal loss . Stealth drones cannot be mixed with any other units in an army. Stealth Drones are also great for quickly transporting commanders to bases. If you keep more than 1200+ drones on a base they will defeat any enemy scouting units every time since the maximum amount an army can be is 1200 units. Keep in mind that you will lose some units each time even if the defence is successful.