draw calls for instantiated Prefabs

hello , i have a lot number of enemy tanks increase every 10 seconds , every tank increases the draw calls 10 , and the game become very slow . how can i solve this problem .
Note : i use combine children but it,s not working , it,s work for another objects

Each object that makes up the tank in combination with separate materials will account for the number of draw calls. moving parts will obviously need to be a separate object, but by combining meshes into a single one, and using a single texture map (or atlas of many images) with a single material for all tanks will reduce draw calls significantly.

However, since you are not providing any references (screen shots of the models or examples of the textures/materials)… it is hard to say exactly, but in general the above holds true.