Obviously there are games with one full game run, and others with a thousand like Smo, but what would you say is a good amount?
At the time of writing, the site has 2 732 073 runs across 26 878, equaling to roughly 102 runs per game (if we were to calculate the average amount of runs for all games on the site).
Realistically, it's impossible to really answer this question. A "good" amount is subjective and what makes one game more popular to speedrun than other is based off many different factors. Some games I moderate have ~20 active runners, some 3, some I'm the only runner for.
It gets even more subjective with quantity when you consider, on certain games, that I could complete a Full-game category and have submissions for 100 ILs if they were short enough.