How do I determine the number of receptacles/lights that I can add on to an existing circuit? I moved into a new house and the garage has one light and one receptacle.

 

There is no restriction in the NEC. It depends on what you intend to use on the circuits.

 

I wanted to add a couple of receptacles and 3 overhead lights

 

It sounds ok. IT all depends on what you want intend to use the recepticles for. If you are going to use several large power tools then it won't be ok. If it is just convenience outlets you won't have a problem.

 

You would have to check on what you intended to connect the receptacles to and how large the wattage is on the lights. Then you'd have to factor in what is already on the circuit. The nect thing would be how large the wire and breaker is if it is #12 wire and a 20 amp breaker then you have more options then #14 and a 15 amp breaker. On the average this would not matter unless you intend to connect some power tools, or a freezer or something else that will really draw power.
Let us know what you are condsidering to install and what is there already and we can give you a better answer. Please include the wire size and breaker size, so we have an idea of what is going on.

 

Although many people would just go ahead and do it, I suggest that you do the math as gard suggested. Add up the wattage of all the existing and planned loads on the circuit. This isn't a particularly easy thing to do, but it's worth it. It's easy to see the wattage of a light bulb since it's printed right on it, but with something like a television, you may have to find the manual or nameplate.

But please consider that you are prohibited by code from adding to a number of circuits in your house. Do not add to a circuit serving kitchen counter receptacles, dining room receptacles, bathroom receptacles, or the laundry area.