IMHO I don't think it's really a numbers answer.

If you're backing only horses in the red you're 100 winners will come much quicker than the guy who backs horses at 20/1 or more. Or if you've backing 4 horses per race per day compared to 2 bets on a Saturday you're number of bets required to assess are over a completely different time [in theory].

It's really time - and in that respect it's a full racing season needed to accurately assess.

Even if you're basing your system purely on numbers with little regard to form - which very few are I'd suspect - you still need the full racing season which exposes you to all facets of form.

Group 1 races, blacktype races, standard races, lower quality races. High quality jockeys...lower quality jockeys...apprentices. Large fields, small fields, dry tracks, wet tracks. First up horses, horses that are well into a campaign. Sprints, middle distances, staying races.

Even if thoughts, ideas, systems choose to drill down on a theory tied to one facet of form you're still likely to be influenced by other factors. eg you may choose to only bet on sprint races in Victoria. But backing horses in May/June is completely different to backing them in say August when horses are resuming for spring campaigns and you start weighing up first up horses with a class edge over race fit types with less class.

Just my thoughts.