If you’re going to run an evangelistic course, don’t wait until you have enough participants. Why would you? If you’ve got two people who are willing to hear the gospel, do it as planned. Just smaller to suit the numbers.
There’s an important principle here; interested people are more valuable* than uninterested people. Basically, if person A signs-up, but then you decide not to run the event, you’re telling person A that person X (who didn’t sign-up) is more important. It’s like you’re saying, “Yeah, thanks for signing up, but people who we REALY wanted can’t come, so we’re going to wait for them.”
Schedule the event, tell people that it’s going to be on, and run it for whoever comes.