The older balers (NH 68 - 271, JD 14T/24T) use 15-19 hp to make the baler run. (They actually store about 50 hp in the flywheel for the bust of power the plunger uses, but can build it up from steady 15hp input.)
Then you need 7-10hp or so to pull the tractor & baler.
Then you need 5-10 hp to pull the hayrack full of hay.
Many people bale with as little as 25hp, tho that is very limiting.
One gets along real well with 40 hp.
The newer balers can use more hp, as they push hay through faster. Probably wouldn't want much less than 45 hp for those, & can use 90+.
In all cases, a live pto makes using a baler much, much, much easier & fun. If you are real short on hp, the live pto becomes more & more important.