A good idea turning bad... (sorry, it's lengthy)

So, according to my darling knot checklist, I should be working on my rehearsal dinner menu.
Once upon a time at the beginning of my planning (ha... man I miss those days sometimes) I had this genious idea to make our rehearsal dinner a welcome dinner for our OOT guests that would be arriving the day before.  Great.
We also decided to host said dinner at the cottage at Glenora, near where the reception will be held.  Awesome.
We are doing a BBQ/cookout w/ burgers, hot dogs, cold salads, etc.  Nothing fancy at all... very casual and informal.  This is a good thing.
I vista printed some super cute invitations to add to our OOT bags, letting the guests know that they are welcome to join us for dinner that night (assuming they will be checking in on Friday)
Insert problem here... me, in my infinite wisdom, set this up so that I will have absolutely NO idea how many people will be in attendance.  Crap.
We have to plan for food, plates, cooking supplies, pretty much everything.  Given that this is not at our home, we won't really be able to just order/make extra stuff and store it.  And, given that this is kind of in the middle of nowhere, it would be really awful to plan for less and not be able to feed our guests.
I was such a spazz about a head count for the reception that I mailed my invitations out without a single peep in them about the welcome dinner.
Now what?  lol  
I still have a ton of time to figure it out... I could always call or email each person traveling (about 100) and ask... but I think in my head I kind of pictured it as a nice surprise... bad bad bad idea.  I am really annoyed that I ignored a detail so important (will I be having 10 people or 100 people at the welcome dinner).