If we assume an average ejaculation is 5 ml, and a bathtub is about 40 gallons, which is about 150 L, you'd need 30,000 ejaculations.
According to Wolfram Alpha, the average human body (sample of 521 people ages 17-51) is about 65 litres, so you can take that out, leaving you with 85 litres left to fill. That's still 17,000 ejaculations, but the number has dropped a lot there.
You could probably take it down by another five litres and still end up with a fairly full bath, which reduces it further down to 16,000 ejaculations.
That's assuming you want the bath to be full, though.
Let's assume that you managed to get 40 guys to agree to take part in this fantasy. Each guy would have to contribute 400 ejaculations (again, assuming they're all average in that regard). If each guy could get two out a day and you had sufficient storage, you'd be able to fill that bath in just under 7 months (or just over 4 months if they can each get three out a day).
Taking MatthewVett's scenario, on the other hand, if you had 8,000 guys coming twice (or around 5300 guys coming three times), you could fill that bath in just over 21 hours. At those kind of numbers your breaking the world's largest gangbangs, what, 10 times over?