I also have been asked this question many times in the past.
In my opinion only.
NFPA 99 only address two items, concerning Reserve Headers.
(2012 Edition)
1) Sufficient number of gas cylinders connections for an average day supply.
See sections: # 5.1.3.5.11.4 + 5.1.3.5.12.4
2) Not fewer than three connections. #5.1.3.5.11.4
This is totally with the understanding, that we are talking about RESERVE Supplies Systems and not Manifolds for Gas Cylinders Without Reverse Supply.
If SO!
1) Sufficient number of gas cylinders connections for an average day supply.
See sections: # 5.1.3.5.10.4
2) Not fewer than two connections. # 5.1.3.5.10.4