abestrange assumed 30 spits per day, I'll see what I can come up with.
Maximum liquid contents for human stomach: On average, the human stomach holds about 1 liter of fluid. However, the human stomach can stretch to hold about a gallon (slightly over, but not by much). So, it's safe to assume we have a gallon to work with.
As abestrange pointed out, zombies likely have an increased rate of Gastric Acid Regeneration (GAR). Also pointing in their favor is the fact that traditional "puker" or "spitter" zombies have distended stomachs, a clear indicator of increased stomach capacity.
Assuming two cup's worth of vomit for each attack, a gallon's worth gives us eight vomits or spits. This would be for a regular zombie that lacks an overgrown stomach, which I assume would hold a lot more fluid; let's say sixteen gallons. That's 128 spits.
If we go for a liter (4.25 cups per liter is a good estimate, it's really 4.22675, or rounded up, 4.23 cups) instead, that's 3 vomits for a regular zombie and 48 spits for a "bloated spitter."
On a final note, what's wrong with biting or scratching? Biting and scratching can do a surprising amount of damage (not speaking from experience here, but I have a pretty good idea of how damaging those are), and besides that, after your zombies run out of fluid, they're going to need another defense/attack/infection method.
Not only would fangs and claws help a lot in making your zombies deadly, but any sane person can and should be freaked out when people are biting and scratching. It's just not natural.