What is the max number of outlets on a GFCI circuit?
There's no limit.  A standard GFCI will protect up to 20 amps, drawn from any combination of receptacles, either the built-in one or any number of additional ones connected to its load terminals.  If you're trying to add GFCI protection to an existing circuit, you needn't worry about how many outlets are downstream on the 'load' side, assuming things are correctly wired presently.  

Now, if you're installing new circuits, from a practical standpoint, you might want to keep the number of outlets per circuit reasonably low, and instead run separate circuits (with their own GFCIs) for additional outlets, not because the GFCI couldn't handle more, but because you might want to use more than 15-20A at a time in a given location.