I've been working on a weapon pack for a while now and I wanted to make something like an orbital cluster-bomb, but I don't know how to make multiple entities drop at the same time. Can someone help?

Use emitters attached to the object that spawns. Each emitter emits a single object on creation.

Alternatively, Lua.

You sir, should receive a helpfulness award. Thanks.

The W40K recomp has a fancy Lua-based fire support system that Asklar coded up, it uses grenades to mark the target location. You could look at that (it's all in the 'Strikes' folder) or ask Asklar about it.