The 3D models in the default library + the ones created by contributors contain 7 staircases, without counting the numerous ones available at DAE/Collada format in Google Warehouse. What models would you miss?
By the way, if you want to change the top view shown in the plan, please have a look to this thread. This is not so difficult to achieve, so feel free to contribute to a new staircases SH3F library.