Could i put my two cents worth and say that i would take it further and say 3airframes. If there was a deployment. Two airframes amongst 4 ships is going to keep the pilots in the air 24/7. As to maintenance and training well we are still at the minimum operating levels at 3. I should say 4 for safety reasons and for less stress on further maintenance on the existing 3. Maybe this is what you meant. Sorry for any confusion.
The new vessel along with the MRV and Frigates are the most likely to be deployed outside the region, it is therefore IMO ideal, as you say Gibbo, to have a Seasprite available for each. The MRV for example is designed to carry 4 NH-90s AND a Seaprite. The ability to carry a Seasprite enables the MRV and the new 'Tanker' to contribute to force protection and projection in a task force environment.
To have one Seasprite for each of the 4 ships plus training and maintenance, would indicate at least one more airframe, preferably 2.