Well, because Active FoxPro Pages are completely written in Microsoft Visual FoxPro (VFP) any limit of VFP counts for AFP, too.
In general the sizes of variables are limited to max. 16MB or arrays can hold only a maximum of 65.000 entries. But in many cases these limits can be ignored due to other solutions to reach the goal.
With upcoming version 9.0 many limitations of current versions will fall.