Have to have at least 3 players "playing" the QB. The 3rd blitzer can be in a spy and still be considered playing the qb. So you could blitz both DE's and have one DT go in spy and it's acceptable.
WHAT YOU CAN'T DO
No blitzing 2 and having d lineman go into REAL coverage. Spy is not REAL coverage, it is playing the QB.
You can't have 2 manual spies.