In my experience 6 co-pilots is about all the BTC's nerves can take
Add a 'safety pilot' on the jump seat to assist the BTC with many things and you have around 7-8.
Each co will get (in my experience and hopefully
) around 6 landings, the last a 'proper' one to a 'stop' and taxy around for the next victim to take the seat. It is a longish day when you add on any transit to and fro and refuelling etc.