If you have multiple proposals, feel free to offer them. That helps us distribute proposals across students in case students propose doing the same project and it's nothing we could split in two (GSoC doesn't want students to work on identical topics because that just doesn't make sense). However, we expect students to provide some amount of detail on their plans (e.g. a schedule that they'll try to follow if the project is accepted), so don't stretch yourself too thin: your chances will be better with 1 or 2 great proposals rather than 5 mediocre ones.

Patrick