A few figures from last year, from their Facebook page:
Based on last year's application numbers of approx 13,000, in the region of 4000 candidates completed Stage 1 in full, with approx 2200 candidates being invited to Stage 2. We held 52 HMRC Assessment Centres last year, with each centre hosting 12 candidates, and therefore 624 candidates attended our centres. There were 250 vacancies for TSP 2015.