I recommend basing the number of invitations you send on the number of meals you have scheduled. A good guideline is:
Invitations Sent = 3 x Meals ScheduledFor example, if you have 10 meals, plan to invite about 30 people.
Why invite so many people? It is likely that a certain number of people you invite will not check their email (infrequent computer users). Others may be away for a few days or on vacation. Some may open and view your Plan, but be unable to bring a meal on any of the days. So by inviting many more people than meals, you are much more likely to find people to bring meals on all the days you scheduled.
It is great to see a schedule of meals with a friend's name next to each one!