Hi
The joys of a common language!
Here, (at least in financial accounts terms) turnover means the amount of income they generate, usually per year but I think in the US a lot of companies report these figures every quarter. Accounts means the numbers they file for total sales, cost of sales, etc.
You probably won't find out how much of their sales are from seminars and how much from other projects but you could do a "best guess" (which is all a business plan is anyway).
Here in the UK, Christopher Howard recently ran a free event which was attended by around 1,000 people. The event was content-filled but was also a "pitch" for his higher priced products. My guess from what I saw is that maybe between 5% and 10% of the attendees went on to spend money on at least some of his next-level seminars, which were priced in the early thousands of pounds. From what I've heard, Anthony Robbins events are bigger. From that, you could probably extrapolate to get an estimate of the overall market size.
There are also some figures on Wikipedia
Self-help - Wikipedia, the free encyclopedia that put the market size at $8.5 billion in 2003. It's a big market!
Good luck with your business plan,
Trevor