No point in me guessing how many we normally use, given how inaccurate your guess was for how many we’re using now.
But someone will know it and be able to calculate the volume needed. If some go out of date on a cyclical basis so be it.
It’s prevention for an identified risk, it’s expensive, in the same way that Trident is for similar reasons.
Of course as I said stockpiling is only necessary until you sort out the actual answer of UK based production
If you’re stockpiling hundreds of millions of them, then the UK manufacturing option goes out the window, as you’d need to buy them as cheaply as possible.