If a rate adjusts once a yr, how much does it adjust by? Is it usually a fractions or entire points?

It depends on the terms in the note.

Typically 1-2% up or down at each adjustment is the cap. In prime credit arms it is normally 1% and in Sub-Prime credit arms it is 2%.

2/2/6 Arm = 2 years fixed/2% + or - at each adjustment/adjusts every 6 months after the fixed period.

Cool guy, please keep in mind that the adjustment is tied to an index + margin [as shown above]. The index is almost always a short-term index, i.e, 1,3,5-Year Treasuries, and even LIBOR.