The way I look at it is: the investment CBS is making in season one of this series is huge. They likely have done cost analysis in what they can pay for a Star Trek series in 2017, what that series needs to be a success (read: not necessarily profit) in advertising, subscriptions and licensing to both foreign markets and actual merchandise. CBS, at the end of the day, is taking, what in their eyes is, an acceptable risk for this series. They know what they're doing. Far more than the armchair network execs who think this series will fall flat on its face. This is a brand new world for Star Trek. And we can't look at success for it the same way as we did with the series of the past.
Discovery will be fine. I would be shocked to not see if get a second season. A third season, I'm a little leery on at this juncture, but we will see for sure!
So, maybe 40 of the same class like Excelsior in TNG.