It depends on your budget and conversion rates. If you get a few conversions per month there is no point in trying to split test 8 different audiences as you’ll learn nothing.
A basic starting point would be to split up the time you keep them in the list, i.e. 7-days, 30-days, 90-days. The actual range will depend on the typical funnel time for you to convert clients.
As for application, if you use automated bidding such as max conversions or tCPA you don’t have to worry about bid adjustments.