Often you are not allowed to park your bike on the street or sidewalk right outside Tokyo train stations at busy times, mainly, mornings and daytime during the week. There might be a lot of bikes parked there but you run the risk of having the city come by with a truck and impounding all the bikes. A bit farther away from the station and you should be fine, or use the local trick of parking your bike out the front of a convenience store or fast food restaurant near the station. There are always bikes out front of these as people pop in to buy something, so lots of people use that to camouflage their all-day parking.