Quantity depends on the guy, his diet, and age. Guys with bigger balls have more capacity, but poor diet can easily negate that, as can several different medical issues. Interestingly, GOOD diet can also effect it. When I went on the Paleo Autoimmune Protocol, I noticed that the total quantity had nearly halved. My bloodwork is absolutely stellar, yet my libido and output are both down. Undoubtedly part of that is due to being quite deeply hypothyroid (low metabolic rate). If I don't smoke, my heart rate runs in the mid 50s, which is a concern for an upcoming surgery in about a month (bradycardia during anesthesia).
As for the spurts, it's due to the rhythmic contractions during orgasm, much like the contractions of a woman's orgasm. It's easier to see on a horse or dog, but you probably get the idea. Everything down there *pulses* several times during the seconds-long orgasm.