Water usage when some sprinkler heads are capped off
The answer is, "it depends", but you will probably be using less water.  If the section the sprinklers were in was overloaded, removing the few heads might just give the rest of the sprinklers extra pressure and allow them to use much more water.  If the section already had good pressure, removing the heads will probably not affect the other heads much and water usage will go down relative to the number of heads removed.

The best way to measure this yourself is to look at the water meter and record the number, then run the sprinkler section for 5 minutes and record the meter reading again.  You can divide the gallons used to get a rough value for "gallons per minute" that the section uses.  This can help you plan your water usage, and even alert you to leaks if your water usage is way off of your estimate.