Thats a bit of an interesting "dilmema" that we also are trying to find out how to get a 100% accurate count since me and my buddy and a bunch of freinds also do a no charge home haunt.
There are a few things we have done in the past each with our issues. Due to the fact of reapeat people coming thru.
At one point (for 2 years)we made a specific ammount of "goodie bags" for the ToTers .. and we kept count that way . that worked OK but obviouisly no the best way to do it . And rather time consuming to do when there is a whole bunch or fo other stuff to do to get ready.
We used a "clicker counter" last year to keep to keep count. Which worked OK unless the person with the counter "clicked" one too many times. Or if the "clicker" malfunctioned, which it acctually did quite a few times.
This year we going to do tickets. We bought a CHEAP roll of raffle tickets,from the local Party City. Each person will get a ticket at the end of the driveway. And before they go into they haunt they turn the ticket in to the greeter at the front door. And it will be put in a bin .. and we wil count them at the end of the night. This seems like the easiest way to get a accurate count even with repeats . Much less room for human or mechanical error.
Again possibly the biggest "drawback" is the "repeat" customers considering the fact there is no charge and unless you have a REALLY good memory you will miss the fact that that same person has been throuh multiple times. If you do have a good memory .. you COULD deny entry to people that have been through more than a few times. But for us ast year we had 350 people (according ot the "clicker") come through between 4:45 - 9 pm on Haloween ONLY (its the only day we do it) and once things get "hectic" it is tough to tell who has been through and who hasnt .. Unless they make it a point to make it obvious they have been through .
Hope those ideas help.