No numbers generally means no development. I take it images don't show up in that part of the roll either? Or they show up but are really thin?

How are you agitating? Assuming you're using a paterson tank or similar, the usual way is to do an inversion every second for 60s at the start, then 10s of inversions at the start of every minute.

Development time/temp/agitation lead to contrast, and you just need to be shown what a "good" level of contrast looks like. If you have completely transparent patches on the film, they will come out completely black, i.e. are under-exposed. Sometimes (e.g. night scenes with unlit areas) this is unavoidable but for most circumstances, it's undesirable. A common fault is to have black hair become an unexposed black hole (clear on the film) with no detail.