I was an apprentice at the time, but REN (Ringer Equivalent Number) numbers came into existence when PO Telephones first allowed people to connect their own devices to the network - part of the introduction of sockets (circa late '70s early 80's), rather than hard wired phones. Phones back then had a bell set and associated coils to operate it, and the the bell circuit was wired in series, whereas phones are in parallel. Due to the current drain of the bell sets, it was determined that a max of 4 (I think) was all that was permitted, due to the power drop in each bell set.
Modern devices do not use the bell circuit - they just sense the incoming ringing current, then electronically make a ringing noise, so then REN number is now totally superfluous and you can have as many devices connected as you like.
