I use slices only on longer (drum) loops. How many slices depends on how long your loop is. If it loops around for 1 bar you might want to try 8 or 16 slices, 16 or 32 slices for 2 bar loops, etc. Play around with what works best on a particular sample.
One thing I like to do is slice drumloops up so that there’s a slice for each step on the sequencer (16 slices per bar), then map them correspondingly (step 1 = slice 1, step 2 = slice 2, etc.), and play around with rearranging the slices to create a new beat. Or throw a random LFO onto the slice/start parameter and you get weird/nice varying drum patterns.
Being new to the OT myself, playing around with the megabreak of doom technique also gave me a bit more insight into how and when to use slices. Check it out:
Megabreak of Doom