To sustain a fleet of 137 jets (7 squadrons, an OCU and an OEU) over the planned (30 year?) life of the jet you need 232 aircraft.
There's attrition, maintenance reserves, etc to take account of. So you need many more than will be in frontline service at once.
Typhoon is replacing F3 (165 procured) and Jag (200 procured) and the RAF intends to keep the aircraft in service longer.
There simply aren't '150 spare jets'