I never set up a dedicated tank to raise the fry. I would venture to say I had easily 1,000's upon 1,000's of newly hatched fry that all became the buffet line for the other fish. My breeding clowns kept up that feverish pace for at least 3-4 years.
If you're looking to only raise the fry from one clutch of eggs then one 8g tank should suffice.....until they start to grow. Once they get bigger, but are still too small to introduce to your main display, you'll need to have either additional tanks or one larger tank to be able to handle the increased bioload.
Your 8g tank should be set up with no inverts or any other livestock. It should be bare bottom, with a heater, hob filter with a sponge over the intake, and a small light. You'll also need a constant supply of rotifers until the clowns get their color, at which point you can change the food.
Russ