You may be able to do this via a "calculate" script that will process all the records passing values into a variable and then write the variable back out to a global number field.
You could attach it to an update button or to a particular script trigger. Without knowing more cant help further.
Sadly all script solutions are out of the question
We'll it seems I solved it myself. It's possible to do a statistical sum on a calculated field (which I can't explain why I thought was impossible). So simply doing a count(related record) and then a summary of that field works.
Sadly it presents a new problem, it's slow. Whereas earlier filemaker server did the calculations it now seems to be done by the client, having about 20k records makes it annoyingly slow, any better solutions?
Maybe, but we'd need to know a lot more about how you've set up your tables and the calculations you need to do to produce the results that you want. Especially when you make statements like: "Sadly all script solutions are out of the question". It might help to explain why they are out of the question as scripts are often the needed "problem solver" for speeding up such calculations.