The answers to these questions are not black and white.
If you look at “hard facts” the average starts per runner calculates to 6.20 starts per runner per year. But that in no way means 6 starts/year is “normal” or provides any sort of benchmark to evaluate a race record against. There is a lot of variation.
The single best thing you can do is ask the connections involved with the horse, but that’s not always practical.
The next best thing you can do is have someone knowledgeable about racing evaluate the entire race record on Equibase (not just the number of races). There are a lot of nuances that an experienced eye can pick up on: type of races, drops in class, prior knowledge of the trainer, gaps due to meets ending, etc.
Speaking in gross generalizations, long layoffs of several months between races are usually (not always) indicative of a problem at that moment in time- especially of the horse was running well or on a regular schedule prior to the layoff. Of course, that does not mean that the problem will still be a problem today. If the horse returned to his old “form” after the layoff, it’s probably a non issue.
But each horse is an individual and you really need to evaluate the animal in front of you, not the paper trail.