A couple of options.
- Have a look at: http://torstatus.blutmagie.de/
If you scroll right to the bottom of the page you'll find a box labelled "Aggregate Network Statistic Summary". In here there's a field for the total number of exit routers. The number of relays is, I believe, in the row showing the total number of routers.
You've already linked to this. You'll find it difficult to get exact figures, but the graph you want is the second in the list: "Relays with Exit, Fast, Guard, Stable, and HSDir flags". The "Running" plot shows all running relays, while the "Exit" plot shows exit nodes.