In addition to finding a definition for "biological concept" be sure to check other relevant help topics:
"How to Ask"
You should only ask practical, answerable questions based on actual problems that you face
Make it relevant to others
The meta of meta:
The opinion of one user about what should and should not be on- and off-topic is not canon. Meta is a good place to share opinions on what should be on- and off-topic, and people can show agreement or disagreement by voting. Please don't target one user or make this about being right and wrong: there's no need to call someone out when you disagree with them, just make your own point known (this is often best done by posting your own answer).
Your specific example:
the reason I am against questions on the number of ribosomes in a cell is that they are not generally about biological concepts.
I personally agree with this sentiment, but I would have phrased it a little differently. In my opinion, there is little value in "trivia-like" questions. This is not a trivia site, and trivia questions tend to take more effort to answer to our standards than they are worth. There tend to be freely available answers all over the poorly-sourced web that are sufficient for the curious, and yet little actually scientific study because these questions tend to not be that biologically important.
To make one of these trivia-like questions on-topic, I would recommend following the two bullets I highlighted above: making questions about actual problems and relevant to others. The key way to do this is to provide context for why the answer is relevant. "I'm curious" by itself isn't a great explanation.
Additionally, I would argue that providing context helps keep a question in the spirit of being about a biological concept, rather than trivia.
Importantly providing additional context helps answers to be more relevant to what the poster is actually asking, avoids XY problems where the asker thinks a piece of information will help them but it really will not, and makes it more likely the question and answer will be useful to future users.
disclaimer: I'm a moderator but I'm not intending this answer to be definitive or to speak for other moderators; I would likely comment on such a question to suggest improvements but would not use my overriding close vote, even though I may have used my close vote when I was a regular user