From the BBC web site:
"The 1922 Committee chairman - who is currently an MP called Graham Brady - does not reveal how many letters he has received until it reaches the 15% mark, which means 48 letters would have to be received.
This would trigger a vote of confidence in Mrs May among all Tory MPs that could happen as soon as the next day. It would be a secret ballot, and if she won, she would be immune from challenge for a year.
But if she lost, there would be a leadership contest, and she would not be able to stand."
I suppose if the MPs have stated they have no confidence in her leadership, they are unlikely to vote for her to be leader in the contest.