The Space Station has two distinct halves, the Russian Operating Segment (ROS), and the US Operating Segement (USOS).
Here is a picture I love of when the ISS had 6 vehicles attached all at once.
The ROS has 4 docking ports, that have accepted:
- Soyuz (Manned crew)
- Progress (Cargo, unmanned)
- ATV (European Cargo vehicle unmanned) - retired
The USOS has two docking ports and two berthing ports.
Using the berthing ports (CanadArm2 grabs them, and positions it on the port) we have:
- HTV - (Japanese cargo vehicle) retired, will be replaced by HTV-X
- Dragon V1 (Cargo, unmanned, retiring soon in favour of Dragon V2 cargo which will dock not berth)
- Cyngus (Cargo unmanned)
Using the docking ports where the vehicle handles docking on its own, and can leave on its own we have:
- Space Shuttle (Retired)
- Dragon Crew (Demo launched, first man flight May 2020)
- CST-100 Starliner
- Dragon Cargo V2 - Will replace Dragon Cargo and dock not berth.
- Dream Chaser Cargo (in the future)
So quite the selection of vehicles have docked at the ISS and more coming in the future. I ignore Starship since it is practically bigger than the ISS on its own, so who docks to whom in that one?
Going back to the picture at the top the HTV-2, berthing port, now has a PMA/IDA combo for docking. The PMA the Space Shuttle is docked to has an IDA added and can be used for docking.
The port directly below the HTV-2 is where Dragon/Cygnus usually berth, with a second port freed up when the moved the grey module sticking down out of the way to free up the second berthing port.
The schedule for visiting vehicles is complex. There is always a crew vehicle that can return the crew it brought. From the end of the Shuttle, that has been Soyuz (crew of three), soon to be supplemented by Dragon Crew (Crew of 4) or CST-100 Starliner (Crew of 4) on station.
There were 5 ATVs, 8 HTV's, and many many Progress, Soyuz, and Shuttle missions, as well as 22 Dragon, and 14 Cygnus missions over the life of the station.
There are usually 3 vehicles at any time (1 Soyuz, 1 Progress, and often a second Progress or Soyuz) and sometimes more. There is no real fixed interval between visiting vehicles but it is a complex dance of scheduling managed by the team that runs the ISS.