Hi, Welcome to Missionchief!
The more Fire, EMS and Police Stations you make the more calls you get. I recommend making as a start, 4 Fire Stations = 4-6 Engines, 2 Rescues, 2 Ladders
,2 or 3 EMS Stations with 2 or 3 Ambos each, and 2 or 3 Police Stations with 2 Patrol Units each until you expand more. Don't expand all at once cause you'll have a hard time catching up with your calls. And be sure you're with an alliance that has hospitals nearby for your ambulance transport requests if you don't already have one.
Then work your way up with adding a Fire Stations once you have enough Engines, Trucks, Rescues, Ambos, Chiefs etc. Go with what your depts in real life have but also don't be afraid to improvise so you have enough. Get creative. Throw your little bit of twist into it. If you have anymore questions feel free to ask away!
Hope this helps,
Rescue