If you are going for an RTS with many units, you should look into
syncronized simulation for the multiplayer.

Its revolves around the concept of only communicating action-commands to the server and back to all clients.

The game itself must make shure that all clients have receive the
SAME actioncommands every game-logic tick.

Then they execute them based on the exact same (using syncronized seeded Random values) manner.

This is the concept used in the Age of Empire games for example.
It enables you to control several hundred RTS units by only
sending a few bytes of actioncommands.

Its not appicable to fast-reaction MP Games (Shooters/Racing)
but to slower reacting games with many entities.

Here a doc on that:

SyncronizedSimulation