Use a spawn script, there are to many available to list. I can only advice you to go through all script download sections and try some scripts which sound good to you.
You need to find a spawn script which only spawns the units when it is really "needed" (as in the player is coming to the location where they should be).
- Arma 3 script section
- Arma 2: Operation Arrowhead script section
- Arma 2 script section