I'm a specialist in this area. There are a bunch of botanical agents to enhance libido, and by virtue of their mechanism of action, they should increase semen volume as well. As with most herbs the daily dosage is usually 2000-4000mg per day for the "whole herb" and 500-1000mg per day of extract. I recommend combining two or more for best effect.
I seriously doubt any herb is going to change semen volume. That volume is controlled by your testosterone levels. You can increase your testosterone levels by moderate resistance training, by being in a winning mood (it's been shown that guys on the winning side of a soccer match have much higher testosterone than ont he losing side, I don't mean the players, I mean the fans!), by getting sufficient sleep, and not eating any sugar/carbs at least 4 hours before bed (that increases your growth hormone which also has androgen effects).
Depression, anger, continued stress, and helplessness are all testosterone killers.
Extracts, especially dry extracts, would be more likely to upset stomache. Some extracts must be taken with food, but it depends on the person. I've observed much variability in stomach sensitivity to high-phenolic type extracts, like green tea or rhodiola.
As to Orion's comments, totally agree with the other factors he mentions. As to the comment on efficacy of my recommendations, I make the assumption (maybe I should not) that increasing general libido and "yang" type energy would increase the semen volume overall, at the end of the day type of thing, cuz your ejaculating more overall. Anyways....can we talk about something else? Before Bob Dole chimes in.
A friend of mine is taking Ogoplex and that seems to work for him. I have seen the difference only because he wanted me to(I'm not gay). I am going to try it myself to see what it does for me. There are lots of websites that sell it, do a search on it.
Poor seman volume can be a sign of a zinc deficiency. You lose zinc and arginine in each ejaculate. I don't think you need the arginine. But your body must have zinc in order to make testosterone. Low volume can be a sign of low testosterone from a zinc deficiency. The highest concentration of zinc in the body is in the prostate.
But I don't recommend that you just take some zinc. Taking zinc alone or even with a multi vitamin can cause another deficiency. You could develop a copper deficiency or a manganese deficiency. Minerals must be balanced. If you do decide to supplement just do some research so you understand what you are doing and don't cause another problem. Or just consult your doctor about this.