obvi charges by the participant – but only while you are sending out assignments. If you don't have active projects, you won't be charged.
Billing begins as soon as you send out your first assignment. This takes your project live, and it will accrue fees until you mark it as closed.
Each active participant adds $25/week (prorated daily) to the fees. For example:
4 participants x 2 weeks x $25/week = $200
Since fees are prorated to the day, you don't need to worry about having to overpay for a partial week.
There's no hard limit, but obvi was designed for focused qualitative research with relatively small groups. While we've done large national projects lasting several months with 30 or more participants, many projects have just a handful.
What's great about obvi is that with no setup, it is easy (and inexpensive!) to start small projects the moment you need them.