10
Something that interests you. If it doesn't interest you, you probably won't finish it. Odds are you'll stop as soon as you hit a snag. But if it interests you, the odds of persevering through the challenges are much higher.
And remember, you learn a lot more pushing past the big obstacles than just coding the bulk.
Blog is something you will hear a lot but if you want something else you can try to do a simple online game (Not flash...) I mean browser online game.
Blog aggregator would be plain easy and interesting for a starter Or some kind of public web services consuming application... Facebook application? :)