I don't like to wean them at less than 4 months old, so she has her own calf plus a very young set-on for at least 4 months, then another pair bought at 4 weeks' old for at least another 3 months.
If the timings all work out and she's handling the workload comfortably, I can just do 4 + 3 + 3 = 10 months lactating rears 6 calves in total. Or perhaps she didn't get back in calf within 3 months, so I've got longer than 12 months between calvings, so can manage a third set.
My most productive cow has been known to manage 3 calves together mid-lactation, but generally 2 at a time - or 4 between 2, as it all gets a bit big happy family now there's more than one at a time in milk - is enough. If it works out they get 3 months dry, that's fine. I don't like them to have less than 2 months dry.
It also gets flexed around lambing, as I keep 1/2 of one cow for milk for the lambs. And sometimes the suckler herd has a waif needs a foster mum, so that has to be factored in when it happens.
And I can sometimes manage to have a new calf come in on a cow nearing the end of her lactation, then switch to another cow within a few weeks - as I said, much depends on timings!
I also have to organise things so that all are well-established in their family groups if I am going to be away for more than a night or two, so that it's as easy and uncomplicated as possible for someone else to look after them all while I'm away. So last year, when I was spending a lot of time in the Midlands with mum, I only put two sets on each cow so they could just be turned out and left out when I was away.