Ideally it would work as you would do, you receive the package you take a couple seeds of the types you would like then add the types you can spare. Ideally as the package progresses the number of types should increase. Again as for the number of seeds it is whatever you can spare, remember as the package moves multiple people will be removing seeds.
Along as everyone is fair I don't feel the need to specify actual quantities. As long as it continues to work I'll stick with the honesty system, if it fails i' ll specify the quantities the following round.
Hope this makes sense.