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.
- https://metrics.torproject.org
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.