When do you *stop* recruiting? I agree with 2 ringers per bell for the maximum size of a band (for 8 bells or less). Actually, not just the maximum size, but I would say that's the ideal size. Less than that and the combination of holidays, illness, other commitments, etc can easily leave you short. More than that and all the members of the band might start feeling surplus to requirements and not get enough rope time at the practices, especially the less experienced, which increases the chances that some of them drift away. If it's considered a problem that too many come to service ringing most weeks, then have a rota and/or "lend" some to neighbouring towers to help them out.
The reason I think you should have more than 1.5 ringers per bell is I've seen bands of that size collapse surprisingly quickly when just a few people stop ringing for whatever reason. You definitely need more ringers than bells to cover absences, of course, and if just 2 or 3 people stop ringing in the same year, you're suddenly in trouble if you aren't already teaching some replacements. I.e. 2 ringers per bell still leaves an adequate 1.5 ringers per bell after some of them have left the band.
So, my recruitment/teaching rule of thumb is: If the band currently has less than 2 ringers per bell then teach just 2 new recruits per year until you do have 2 ringers per bell. This way, there isn't a sudden influx of lots of new people, which can be a little disruptive to the social cohesion and ringing quality of the existing band, and the 2 recruits can have more time and attention focussed on them than if they were part of larger cohort. Hence they should progress quicker and be good steady ringers ready to help the next recruits.
If you're lucky enough to have more than 2 people wanting to learn, ask them if they wouldn't mind joining the waiting list and explain how it's better to teach, for example, 2 at a time for one year than 4 at a time for two years. The more learners you have at the same time, the slower the progress will be for each of them, which might also lead to a retention problem. Even if you're starting a new band from scratch, just teach 2 at a time until they can control a bell well and then teach the next 2. Arrange for visitors to come to the practices to support the first 2, then 4, then 6...
This is highly generalised, of course. Local circumstances and the aptitude of the recruits could well lead to very different timescales but I would strongly urge sticking to the 2 at a time principle. People will probably be prepared to wait if they understand the reasons for it.
The above advice comes from my experience of starting a new band from scratch at an eight bell tower in 1981 and then being Tower Captain there for 32 years until I moved away. There were 16 ringers left in the band after my ringing family of four relocated and the band is still going strong to this day.