I have actually made an app for that!
https://rackfinder.app

It is better to group up racks that are close together that you might consider a "clump". This will reduce clutter on the map and is easier for data queries. Then use the capacity tag to specify how many bikes can fit there. I usually mark the center of the row of stands for the node marker. You can however use an "area" way to draw the boundary of the bike parking area. Only a few are tagged this way but it is valid.