Funny you bring this up...I had this dilema today. Not getting married until 4/4 09 but wanted to start buying the little things as FI does not think we need to do OOT bags (thinks they are a waste of $). So, long story short...first trip to Target and I had to make 2 decisions: 1. am I doing OOT bags even though FI says no and 2. if I am, how many do I need and should I start now. I have sent out 71 invites. Thanks to The knot, I know that means 41 invitations due to couples and singles. I am expecting about 50% of that so I assumed 36 people. I played it safe with 15 couples and 6 singles so I bought 21 of all the small items. I figure this way, if I am short...I can find one or two items to make up or if I am over, it will only be by one or two and I can always use Aleve & shout....
Not sure if this helps, but look at your guest invite, the # booked, the # promised, and the # for sure not attending and get a close base off that. Not all of them (if you are short) have to be 100% the same. I know my Gma does not need the aleve or tylenol so if I miss that I can throw something else in last minute.