Assuming you are using plywood, I've found it takes lots - depends on the curve of the deck. You could probably get by with about half as many.

I picked up a bunch of Chinese knock off spring clamps at one of the big box stores for less than half the price. Not great-not as strong, but they work. You will probably need a few small C-clamps to get enough pressure to twist parts of it into place.

I also glued up the layers with tape over the deck. You can then remove it and clean up the stack and radius the edges much more easily before attaching it to the deck. You can also shave down the bottom or the ring in places to get a more even gap for the skirt.

Try it without the glue first. If it's your first time you might want to do a layer at a time - it can get a bit messy doing it all at once.