Alice,
Namaste :)
I'm sure you'll encounter differing opinions on this one, but personally I think it depends on how long the list is! :) I think if you send individually then you are more mindful about focusing on the recipient and the Reiki. To use a very general example not related to Reiki in particular... if you were in room of 20 children it would be very difficult to give each of them the individual attention that they need whereas if you were in a room with an individual child or a small group then it would be much easier to see what their individual needs were and to respond accordingly. I have tried in the past using Reiki boxes and lists because I wanted to send Reiki to everyone that requested it, but in the end I felt that I wasn't being very effective and so these days I limit my distance work to a very small number at a time. Sometimes less is more as the saying goes! :))
Maybe in the end it is just what feels right for each of us as individuals.
Gentle Blessings
Helen