You can allocate more than one unit to one incident. And often, you should
If you just started playing, your instinct may be to send the "bare minimum" response, just one unit to the correct type, to keep other resources available for other incidents. This is actually NOT an optimum response.
Sending more units to an incident actually speeds up the response, so the incident goes off the map faster. This applies to all incident types. Obviously, you don't need 6 police units at a single traffic stop, but having two or three units on scene does speed up the entire response, leaving the map clear for other incidents to pop up. Thus, if possible, send more than one unit to each incident.
Corollary: You can allocate more than one type of unit to one incident
If you've played a while, you've probably seen how one type of incident often turn into combination incidents, like a fire turn into a fire/injury, or injury turn into injury/police. Thus, it's not a bad idea to almost always send multiple unit types to a single incident if they are available.
Study the map for choke points for each region of the map for unit pre-positioning
Some maps have natural choke points for some regions. Divide your various units accordingly for optimum coverage of the area. It's not just geographical, but the road networks, that should dictate where the units go.
More units is more important than better units, at least initially
Try to add one unit every 24-48 hours game time.
If you don't feel you run out of units during the day, add better equipment to the crew of the various units. Try to make all units have the same equipment.
If you add units, don't forget to add crew and equipment, which would obviously cost more.
Send the right unit sub-type, if you just send one
Almost all ambulances are about the same (except air ambulance, but that's different class), but police and fire have different sub-types.
Police have sedan, van, and motorcycle. Obviously, motorcycle is best for chases, but it can't carry criminals back to the base. Van is slow, but can carry several criminals. Sedan is average, and can carry 2 criminals. In general, send a sedan, with motorcycle as backup, and van as second backup.
Watch your units for injuries at end of every day, and hire replacements as needed
All units can operate with a single crew, but it's not optimal. Hire replacements as needed, based on available candidate's skills. Remember to give them the equipment if you standardize outfit.