Is 10 a new limit? Because I'm pretty sure there were more than 10 of us on my KTTK tour back in September. Also, I was able to book it 2 months in advance and still get a spot.
i don't remember many more than 10 when we took the tour twice. But we could have not had a full tour both times.
I think the issue with the OP could be trying to book just a few days in advance. I was surprised I was able to get a spot for Wild Africa Trek 2 days before I wanted to do it.