Well, no-one has regged through me...
what's probably good for starters is people sending me an email at firstname.lastname@example.org
and saying they are definite or thinking seriously about it... i can tot up numbers then.
I got a reply last night, so 8 places can be reserved without names. I will do this. But if i get an idea of numbers for almost certain, I can see if i can hold more places. I will push for holding 20 to start with.