50 twin-engine 787's - One hundred installed T1000's. If retrofit is needed, it will also no doubt include spare engines. Any Idea how many spares?
I presume that it's OK to fly if no more than one engine per flight is at high risk
If a couple come off every day and a spare goes on per aircraft it doesn't seem like any more spares than usual are needed.
In the end the risk analysis should ensure that its extremely unlikely that more than one engine loses power or has to be shut down in the life of the retrofit program.
On the other hand if one becomes too aggressive in swapping out engines like two at a time. human error becomes a more significant risk