It is actually 4 - three to set a scrum and one to feed it

it's 5, it says so in the laws of the game.

...and one in the back line. So 5.

Bizarrely its not actually written anywhere. I have numerous detailed copies of these from my reffing days. I recall a refs society debate on it a few years ago as no-one could find the law written anywhere. The reality is a game would probably be abandoned long before 9 players from one side could be dismissed.

I've deffo read 5 players the other day but can't find the reference, just had another look and BARLA states 5.
RLWC 2013 rules stated 9 as a minimum but included in that number are sin binned/blood bin players off the field but not those sent off -so you could have 7 on the pitch and two in the bin as an example.

British Universities states that a game may start with 9 players aside and they specifically say that this is an RFL law, it does not mention what the minimum number is once play is underway.

It’s as clear as mud. Like I say, a game would be abandoned long before 8/9 could be dismissed