That depends on levels, mechanical skill and perhaps other factors, I'd say.
I believe it's close to not doable (if not doable at all) to do it solo. It would be very hard to go past the first part (Rookgaard), maybe possible on Thais, very problematic on Demon Oak because of the amount of creatures, very very hard on Ferumbras because of lack of damage.
Five sometimes feels like an overkill because you can have someone just standing next to the CM diminishing their "damage" stacks, however Ferumbras Tower is rather slow already with five people, so you can risk running out of time and getting kicking.
My final answer would be 4 players minimum and ideally 5 considering the time limit is short and performing tasks in general with less support from teammates is problematic.