There probably are at least 54 who would put in a letter, but are there enough who would vote against him in the subsequent leadership contest? That would be the concern I guess - do it now and if he wins, they can't get rid for at least another year.Yes of course but let's ask the question and see who does what.
54 letters is what we need but are there 54 Tories who will put Country before Johnson ?
I suspect many will be holding out to see how the council elections go.