Not really. Retired generals and admirals are a dime a dozen. There are 900 active duty flag billets with the five services, 15-20% of those retire each year (there are literally thousands of retired generals and admirals to pick from).
Also, keep in mind that our military today is far too top heavy with flag officers; we DO NOT need 900 active duty flag officers.
Our current level of flag officers, for a military of 1,400,000, is almost the same that we had during WWII, when our military was 12,000,000 strong.
Thank you for clarifying that.