Was thinking further on this and recalled a discussion from many years ago at a certain unnamed 'other' forum where it was stated with sound reasoning that the practical upper limit for one player was fifty 'units' in a game - whether the unit be a single figure or a stand of figures. That jibes reasonably well with the 8-12 units mentioned above as long as the aggregate total is in that 'fifty' range (8 units at 6 stands per unit = 48 being an example, add a couple generals and = 50 , or could be 12 units at 4 stands per unit).
So, I guess, for me, building an army with an upper limit of 300 'units' per side is a good target, maybe a few extra options for varied scenarios. That is for a maximum of six players with four players being the more common likely number. So, skirmish collection max 600, 3 figures per base collections (my norm for multi-based) max is 1800 figures, plus those 'few extra options'. That is, of course, for two sided collections (collection = specific period of history). Most games would be smaller, often times a good deal smaller - especially skirmish games.
As suggested, there is never going to be a 'hard and fast' rule but I think this is a good guideline to keep in mind when determining how many figures to acquire in building a collection. Had I adhered to that rule from the beginning I'd probably have at least 1/3 fewer figures than I do. But making serious efforts to 'down design' all of my excessive collections - and even most of my smaller ones as well.
And truly hope this helps some people avoid their own excess - time and resources that can be put towards other elements of life that are also enjoyed.