A 5x9x8 room = 360 cubic feet. 1 cubic foot = ~7.48 US gallons So, a 5x9x8 room can hold just under 2693 gallons.
The Internet estimates the average volume of ejaculation to be between 2 and 5ml, so I'll go with 3.5. There are approximately 3,785 milliliters to a gallon, so for one gallon of semen you'd need about 1,081 men.
For a 360-gallon room, that means 2,911,133 men.
(I doubt PBS envisioned this particular application of the math skills I learned from Square One.)