This site lists 129 Conservative MPs as having supported Leave.
Robert Syms has organised a letter of support for David Cameron which as I write has been signed by 86 of them.
That leaves 43 who haven’t signed it (some may not have been asked to; some may have missed Syms’s invitation; and some of course won’t want to add their names).
Around 50 letters are required for a leadership challenge. I don’t see one in these numbers – let alone a successful one.
P.S: The challenge scenario probably applies only in the event of a Remain vote. In the event of a Leave one I would expect Cameron to stay on as Prime Minister and Party leader only briefly.