For personal work in 35mm, I usually process four or eight rolls of 35mm at a time in one tank. Sometimes, if I have a lot of film to process I'll load two tanks and run them 60 seconds apart, so I can basically do 16 rolls of 35mm at once. Occasionally I'll process only one or two rolls at a time, but this always feels like a waste of time and chemistry.
I used to process film at a small boutique b&w lab in NYC; I now run my own small lab for some of the same clients, and my processing is done the same way. I use baskets that can process 18 rolls of 120 or 220 at a time in deep tanks. When I have big orders I will load up two of these at once and run them in succession- so that once the the first basket goes into the fix, the second goes into the developer. This means I can process up to 36 rolls of 120 or 220 in a single lights-out session which may last 45 minutes to an hour, including time to load the film.
For sheet films I have several different processes, depending upon the format and the quantity I have to process. For 4x5, I'll run 10 sheets at a time in single sheet holders in small tanks, or 24 sheets at a time, in 6 holders which each take 4 sheets at a time- these go in the same deep tanks as the medium format baskets. For film larger than 4x5, I use open trays. With 5x7 and 8x10 film, I try to limit it to no more than 6 sheets at a time in a single tray; larger films really need even fewer sheets at a time. I can process sheet film up to 20x24 inches this way- though I've found that film above 11x14 should really be done one or maybe two sheets at a time. This is slow and tedious stuff, and very expensive- but man, do they look good!