Hi Peter. As you’ve probably discovered by now the answer to your question is that this percentage is constantly changing! As fitness trends, the economy, and other ‘social’ factors change and influence people’s decisions, so goes the breakdown of one-on-one training versus small group training. A lot also has to do with the particular training niche that a trainer is working with. For example the percentage of seniors participating in small group training may be a lot higher than that for elite athletes (just an example); so take a look at niches as well as numbers.
I hope that this helps.
There was a survey pertaining to your question not too long ago at the IDEA Fitness Journal. Here is the link.