Your limit is at least $2,200 in a 12-month period. This applies to regulation* crowdfunding platforms.
Regulations established by FINRA limit the amount that individual investors may invest across all regulation crowdfunding platforms in a 12-month period, not only the amount on Mainvest. Because MainVest does not know how much you have invested on other platforms, it is your responsibility to ensure compliance with these regulations.
See Section 2.b Investors Subject To Limits on the SEC's website to calculate your investment limit.
*Kickstarter, IndieGogo, Patreon, GoFundMe, Robin Hood, E-Trade, Wealthfront, and Betterment are not regulation crowdfunding platforms.