I think you have to be familiar with the towpath in question. Some are popular cycle routes, where a few more bikes won't cause undue problems. Some are busy with pedestrians/anglers/boatists and patently unsuitable for more than small numbers of extremely patient cyclists. Some are remote tracks miles from anything interesting where the surface may be a bit technical, but cycling isn't going to cause conflict. Blind bridges, tunnels and Silly Sustrans Gates cause bottlenecks as soon as anyone's going against the flow. That sort of thing.
And of course there's some potential for manipulation of the flow by where the towpath appears in a route. Early on and you'll get large bunches of cyclists, towards the end and you may get stragglers in a hurry.
Personally, I think much of this applies to other off-road routes (bridleways, railway paths and the like). IMHO time limits should be generous enough that riders can always slow down for hikers, pootlers, dog-emptiers and so on.