As far as the ideal number of samples to set it to, it really depends on what you're doing. For me, when I'm playing guitar using input monitoring with Guitar Rig or TH2, or when I'm playing softsynths like Kontakt or Z3TA+2, a setting of 128 is actually very playable, so that's probably a good starting point. Or you can obviously start as low as you can go, then just raise it as a project gets larger. I just generally leave mine at 128 most of the time and only raise it if I need to (like when mixing a lot of tracks).