Some ex4200 need to be kept to replace the OOB (mgmt) switches (as many as we have OOB switches (+ a couple spares).
The EX4550's are not needed.
Is 43 a typo? that sounds like a lot.
EDIT: from IRC, it's 3,
As long as we have EX4550 in production, we should keep at least 1 spare, the others are not needed.