OK, now we're going to have to make some pretty wide estimations here, firstly because there is limited data on the subject and secondly because I don't want to spend too long doing maths related to you weird cum swimming pool OP!

Firstly we know there are around 7,666,000,000 people on Earth and whilst slightly inaccurate the best we can do is assume a 50:50 gender ratio split.

7,666,000,000 / 2 = 3,833,000,000  

So there are about 3,833,000,000 men on Earth. Now the most recent data I could find was 2016 so we'll run with that, the global average life expectancy for men was 68. So here comes the next very rough calculation, we'll assume an even distribution between of the population between the ages.

3,833,000,000 / 68 = 56,367,647   

So there are around 56,367,647 men alive for each of the 68 years they'll live for. Now the average age globally for boys to enter puberty globally is around 12, with puberty lasting 4 years and ejaculation starting midway through. So if we assume "midway" through four years is two years, and add that to the average age of starting puberty.

12 + 2 = 14  

So we know the first fourteen years worth of the population are ineligible since they can't ejaculate to help fill your disgusting pool! So let's remove them from the 68 years worth of candidates.

68 - 14 = 54  

Now we need to multiply the accepted 54 years worth of people back up by the number per year we calculated earlier.

54 x 56,367,647 = 3,043,852,938  

Now of these there are obviously some who will be unable to help due to medical conditions. The most common condition relating to ejaculation is premature ejaculation, obviously that we won't exclude since as long it's not so premature they can't even get to the edge of this sick hot tub you're planning to build it will only serve to get the job done quicker! However delayed ejaculation and retrograde ejaculation will hinder your operation. The best I could find for this is delayed ejaculation effects around 3% of the population... However of this 3% the condition only about half are unable to ejaculate at all... So 1.5%. Retrograde ejaculation didn't have an incidence rate and was only described 'much rarer' likewise I couldn't find statistics for other things such as testicles lost to injury, genetics, or things like cancer treatment. So I propose saying two percent combined... The 1.5% we know, plus an extra 0.5% for the 'rare' conditions I couldn't find data for.

(3,043,852,938 / 100) x 98 = 2,982,975,879  

So you have 2,982,975,879 dudes capable of topping up dirty man-splooged pond of gayness (Obviously 'capable' is my maths part, 'willing' is up to your persuasiveness you sick fuck) Now the average volume of ejaculate per person per orgasm ranges from 2ml to 5ml (There are conditions which lead to far lower amounts and far higher amounts, but these have fairly equal incidence so balance each out when looking at average volume)

(2 + 5) / 2 = 3.5  

So we are looking at an average of 3.5ml per person for each of your 2,982,975,879 dudes.

2,982,975,879 x 3.5 = 10,440,415,577 (ml)  

OK, so a lot of millilitres, lets drop it down to litres...

10,440,415,577 / 1000 = 10,440,416 (l)  

So around ten and a half million litres. But you wanted gallons so lets switch it over to weird american old timey measurements...

10,440,416 x 0.22 = 2,296,892  

So about 2.3 million gallons for your sticky jizz dream jacuzzi OP. And to put that in perspective an Olympic swimming pool (50m / 164ft x 25m / 85ft x 2m / 6ft) has a volume of around 660,253 gallons.

2,296,892 / 660,253 = 3.48  

Just under three and a half... So you could fill three full sized Olympic pools, and have enough left over to steam up the sauna, getting the jacuzzi bubbling, fill a splash pool for the kids, etc. etc.

And of course that is assuming only one ejaculation per person, I'm sure plenty of guys could manage more, although it depends on whether you're planning a one time spunky festival cum pool party or whether this is a permanent addition to your house since recovery time increases with age, the average teen might be topping you off a few times a day but the average pensioner will be slapping his weary dick around once a fortnight, so if you're planning long term there are more calculations we need to add.

Anyway best of luck with your plan OP, but keep me out of it!