Regarding using number for inspiration, I recently watched an Charlie Rose interview with artist Brice Marden http://video.google.com/videoplay?docid=983257097746078134
where Brice talks about using numbers given him by an astrology to base his painting off of. I found the interview very refreshing and inspiring, so I thought you might like to watch it too.
I would gather all the art you have done in the past and try to find a common theme in all of it. It's hard to suggest a theme, because it's so personal to the artist. It's also helpful to get some outside feedback on your previous art. Your peers, mentors, friends, and family may see something that maybe you didn't see before. Then you might be able to get a theme off of their reflection on your art.
Instead of worrying about getting bored with a theme, think of ways that you can challenge yourself. For example, I recently finished a series of 100+ drawing over the past few months all inspired by a coffee cup. While in the beginning the drawings were more observational and representational, the most recent drawings were not so much inspired by the object its self, but by a circle, a void, a hole, and/or a window. While you may struggle at times to think of new directions to go based on your theme, use your own personal experiences in life to guide you. Never stop experimenting with materials.
American Artist Charles Demuth (sp?) used numbers more literally in his work. You may find it inspiring to google some of what he has done.
I find it helpful to surround yourself with artists and art. Look up a bunch of artists, or google some art schools. Usually art schools have links to student work, which may be inspirational. I hope this helps. I wish i could tell you what theme to do, but I find the choice to be too personal. Go with you instinct. If it tells you the number 5, or hats, go with it. Good luck!