The way to know is start a few projects + see.

Likely best to only start one project at a time... meaning only one crowdfunding site running the project at a time.

So use different sites in sequence, rather than simultaneously.

And... There's a far better way to do this... at least as a test.

1) Come up with something to sell.

2) Open a http://Meetup.com group with fitness as the category.

This might not seem so powerful + here's a tip.

When you start a Meetup group, once your group is approved by the Meetup Trolls (dark beings who issue such approvals), sometime in the next few days, they will send an email to every Meetup member camped on the fitness category.

You can't find out the numbers any more, because Meetup stupidly (was one of the best marketing tools on the planet) turned this off several years ago.

And, just so happens, last time I helped someone setup a fitness related Meetup group in Austin, TX there were...

15,000 Meetup members in Austin, camped on this topic.

So, when you setup your Meetup group, set the zipcode of your Meetup group to the closest zipcode to you which has the highest population density.