7
I was thinking 5 20mA LEDs would draw 100mA so if I had 2 outputs live the draw would be 200mA. total.
Figure 14 on the Texas Instruments datasheet is the most important information to refer to. There is a limited current available and an issue with overheating if too much current is drawn. That is why the x axis of the graph is the "duty cycle" - the % time that the current is flowing. If only one output is on and it is properly on, that is, not being driven by p.w.m. or suchlike, then the graph recommends that it should not draw more than about 330mA. If you have two switched on, the total current flow should not 200mA per pin. And so forth. So you should be fine with 4 outputs at 100mA each, all on at the same time, according to the graph.
I have always thought it a little misleading that the introduction section states 500mA per pin - as if the chip could drive that through every pin! Figure 14 also shows that the 500mA figure is for 50% duty cycle, not 100%. I think a lot of people run these chips well over the rating and it seems that in many cases they get away with it, but you can certainly burn them out if you chance your luck. Figure 14 shows the parameters for safe and long-term operation.