we have some of the lifetimes ones - four, I think - so with our park tickets, and the daily fastpass (one per guest) given to disney hotel guests, it was entirely possible for the two of us to hold eight fast passes at any one time.
We never did - it just wasn't possible to get through them all!
Best used on very very popular rides - stuff all the fastpasses in, get at least three rides each, just go round and round and round then that's that done in about a quarter of the time it would take to q up. Eg Tower of Terror - I queued for the fastpass machines last time, and got three sets of fastpasses out....we went and had a coffee, then rode ToT three times in succession, then that was that!
We barely had to use them on weekdays - more useful on weekends, when park is busier. (obvious point, sorry)