Be a bit difficult to form up the signatory Diamond Nine with only five, and to give the Reds that many Typhoons the MoD would need to disband a squadron, wouldn’t they?

How come no one is putting up the Lightning II as a potential Arrow mount?;o)

If the Hawk T1’s are broken (but to expensive to fix), if there are not enough Hawk T2 for the Red Arrows, then using a smaller number of Typhoon could be the answer.
I understand the maths, but lets not forget the older Typhoons are already in storage, ie they are surplus and only required as replacement for loses at squadron level, problem is they are an older specification so would require expensive upgrade for front line use. Maintaining five additional Typhoons for the Red Arrows suddenly does not look so expensive. Obviously the cost per flying hour is greater than the Hawk, answer is simply to reduce flying hours and the number of events attended. Sadly the ‘big show’ and diamond nine loops and rolls would be a thing of the past, more likely a solo display by Red 1, a few synchro pair passes with a couple of four ship passes.
One further advantage of using four Typhoons for the Red Arrows is that it would allow the rest of the ‘standard’ squadron Typhoon pilots to joint in for Royal events, trooping the colour etc as required.