do you believe they'll be able to sell 830 new full-year subscriptions a month?
Yes.
That's really not a lot. LA has 13M people in its Metro area, and 900k weekday riders on the Metro system (not sure about the subway itself, but rail is allot 1/4 to 1/3 of that). VPNs have been getting more popular with ads and sponsorships across social media.
So I absolutely don't think it's unreasonable that they'll pick up ~1k new subscribers per month while this ad campaign runs. I don't know how many users they have (it's antithetical for them to release that), but surely they can afford to run a small campaign like this, just given how many servers they have and some back of the napkin estimates to guess how many users would be needed to justify that number of servers.