Question : Problem: computer & monitor circuit loads

If I have a specific computer model and monitor model, what equation or formula must I use to find out how many of these devices can be connected to a 15 amp circuit?

Answer : Problem: computer & monitor circuit loads

You simply need to look at the power requirements for each device.

If the requirements are stated in watts, you'll need to convert them to amps, by dividing by the voltage.   For example, 300 watts = 300/110 = 2.73 amps.

Note that the size of a computer's power supply is NOT the total current draw, since power supplies do not operate at 100% efficiency.   The better PSU's are about 80% efficient;  but many are only 70% (or even a bit less).  You'll be relatively safe if you assume 70% ==> so, for example, if you have a 350 watt PSU, you can assume it won't draw more than 350/0.70 = 500 watts, which is 500/110 = 4.55 amps.

... and, as I assume is apparent, you simply add up the amp requirements of all your equipment and be sure it doesn't exceed 15 amps (for a 15 amp circuit).