Power Functions battery box and receiver limits
The present line of battery boxes all provide 800mA of current and have overload protection to ensure that this level is not exceeded. Here's what the LEGO website has to say about powering motors:


  As a rule of thumb, you can drive 2 Power Functions XL-Motors, 3 Power Functions Train Motors or 4 Power Functions M-Motors at the same time from one Power Functions Battery Box.


That's the basic high-level view. LEGO provides more details about each motor. You can check these out on the element specs page. Here's a list providing the current consumption for each motor type under "normal conditions":


Servo - 150mA
M - 150mA
L - 225mA
XL - 300mA
Train - 225mA
Lights - Negligible ("You can generally power as many as you need in addition to the motors")


So, in order to stay within TLG's recommendations, you shouldn't be running anything totalling more than 800mA from the above list off of a single battery box. Also be aware that if you are doing things which require a lot of mechanical power, such as heavy lifting, your motors will draw additional power. A stalled XL motor will overload the battery box by itself, for example.

As far as I know, overloading the battery box doesn't cause any long term problems, so there isn't an issue with testing your configuration to see if the battery box can handle it.