Probably a ton of them.
I remember an article in a Swedish paper when a footballer in our top league was revealed to have been betting on games. He said something along the line of everyone betting on something. Maybe not football but almost every player in the league bets on something because they have nothing better to do with their time and money.
So I would not be surprised if there is a similar problem in the English game as well.
No ref has ever been caught in England. I wonder if they are monitored in the same way. Probably not knowing football.