There is a very large number. My guess is 300+.
If you want an exact number I'm sorry but I can't help. It will be difficult to quantify because lots of large companies have their own centres which are not advertised, then there's the police, army, navy etc ............................
AS Mike says, there are loads of training centres if you count all the independant driving schools. However, there are a smaller number of test centres. These are attached to LGV Test Stations (which I think there are about 100 throughout the country). This is because when you have your test you need a large area to do your manouvering test and brake control test.