I think you're giving away too much for free. How about 1 mockup for free, not 5. Or how about you just let me pay you $x for 3 months of usage (which will coincide with Project Y that I'm working on). If you let me do 5 mockups for free there's not much incentive for me to pay.
The Pro plan will eventually include additional features which the Free plan doesn't have.
Also, since you can share your designs, there's a benefit in letting people create several for free. There's a balance there, but I think 5 is a good fit for light users.
