I think Boris won't declare until he knows he's definitely got 100+
If he doesn't reach the magic number I expect him to say that he never intended to stand anyway, rather than face the humiliation of realising that he's not as popular as he thought he was. Why else would he have come back from his holidays if it wasn't to be on the ground so to speak?
That then leaves all his backers high and dry (which will make me laugh for a very long time). Looking through the names of his backers, they are definitely the representation of the worst of the Tory party.
My sympathy levels are zero.