Seeing as Wheels, as usual, will not give a straight answer to a straight question, I'll have a go.
At the moment there are 56 Lib Dem MPs. It is widely predicted that they will lose some of these, and not gain many new seats, if any. But the Lib Dems have in the past proved to be very good at hanging on to what they already hold, even in adverse circumstances (e.g. at the Eastleigh by election last year). And in those seats (there are many) where the Tories come second, the Tories are likely to lose a few votes to UKIP. So incumbent Lib Dem MPs, even if they lose some votes, could still scrape home, thanks to Mr Farage!
My prediction is 39.