Well yeah that wouldn't be too many at all, but then going back to what Marcus and EGOL said you're much better off using bold tags for readability reasons than simply using it on keywords really. It has little to no impact in terms of rankings (probably a tiny bit) but can have a much greater impact on keeping your visitors moving forward and converting if used to break the content up into more digestible chunks with a good scent of what's important to them and where their eyes should go next.

I'd probably use bold a few more times in a 500 word piece but use it as what it is, an emphasis tag Not emphasising keywords for Google but whatever it is in the content that will encourage the user to feel they're in the right place... if that happens to combine with some keywords or keyword phrases then great, but don't see it as a game-changer for rankings

I suppose if you were going to look at it as "how many is too many" though it would be a percentage density instead of an actual number because it would depend on the size of the piece.

If you had a 1,000 word page then 50 words in bold might look okay but if it were a 100 word page then 50 words in bold would be ridiculous.