My understanding is that it varies greatly from person to person. Some maintain complete sexual functionality on HRT, some lose all functionality, some are somewhere in between. Sperm production, I believe, should decrease dramatically, if you don't become completely sterile. I have known MTFs who had been on HRT long-term and could still produce sperm (as evidenced by her accidentally getting her girlfriend pregnant). As for how much ejaculate you can produce, I'm not sure about what the general changes with that are.
You'll just have to wait and see how it affects you.