how to know how many bits a microcontroller has?
With PICs in particular, sometimes you find what you need in the Family Reference Manual as opposed to the data sheet.  In this case, the mid-range family ref manual has a chapter on architecture that you might find of use, and its in a different tone than the chip's datasheet.  

For PICs datasheets tend to have a ton of chip specific info,but the Family Ref Manuals tend to show a more global picture.