... and it is also very easy to swap a single connector for a double to give you another ground or a power connection.

Sorry, I cannot help with the number required.  However, with a new harness I suggest not re-using any of the old couplers.  The old parts tend to corrode and crack into multiple pieces.  It is also beneficial to apply a thin film of dielectric grease to the connectors before assembly to keep moisture away from the connections.  While our host probably sells the 2 and 4 bullet connectors, you can also buy them from sources like britishwiring.com

Does anyone know, from replacing an entire harness, how many of the single and double bullet butt connectors is required on a 67' mk1 Cooper S?