How often do monks get nocturnal emissions?
In my opinion, there is nothing wrong with having nocturnal emissions. It's a completely natural physiological function. 

I even dare to say that nocturnal emissions should happen in young and healthy males. If you'd have no emissions, it would mean that you have too low testosterone, testicular disease or other problems.

You say you have no "impure" thoughts, therefore there is no reason for you to feel regret or remorse. Quite the opposite, be grateful that your body is healthy and working well.

Please, don't worry about it. Many ascetics are too harsh on themselves and are trying to stop their desires or bodily functions by practising extreme fasting or some even emasculate themselves. These things are extreme and quite opposite to the Middle Path which Buddha taught. Don't damage yourself, be compassionate to yourself.

On the other hand, there is another extremity, which is the commitment to sensual pleasures, but I think it's not your case.

Peace