In theory, thousands and thousands since each light only takes a small amount of energy (http://en.wikipedia.org/wiki/Milliamp)

In practice it is very dangerous to overload an electrical outlet with too many lights. The often low quality cabling for the lights usually can not withstand the added heat and current for many lights.

We have plugged in 4-5 strands of 100 light cables without any problems into a single outlet. I wouldn't go much beyond that.

Also, keep in mind a single outlet is usually tied to many other outlets all running from one circuit. So that single outlet may also be shared with your TV, microwave, etc.

Safety first.
Source(s):
http://en.wikipedia.org/wiki/Milliamp

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I was able to run 3 strands (75 lights total) together before blowing one of the fuses in the plugs. Moved to LED light strands. Same brightness, but can run 43 strands together (one strand only drinks a few watts).
Source(s):
Philips LED light box

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Here's how to figure it:

Each electrical circuit in your home has a fuse or circuit breaker, typically rated at 15 Amperes (Amps).

Lights are rated in Watts, and small incandescent Christmas bulbs are usually 2.5 to 3 watts each, while the large, C7 bulbs are 7 watts each. The new LED lights are 4 to 8 watts for a whole string of 100, but are much less bright.

Watts = Amps/Volts, so for a 15 Amp circuit at 110 volts, you could in theory power up to 1650 Watts (15*110) without overloading the circuit, though a margin of at least 10% is recommended for safety, so call it 1500 watts even as the capacity per circuit.

Note that extension cords and timers are marked with a rating in Watts and can dangerously overheat if overloaded.
Source(s):
Wikipedia: Ohm's Law

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