As a recent reference ET 99 utilised 6 hueys to support a Bn sized deployment (arguably the largest we would realistically do and even then not without issues so not likely to be repeated anytime soon) in the surge dropping to 4 after everyone was established so comparitively 3 NH90s would be needed to support our largest deployment (throw in a lone A109 for SAR, overwatch, contingency etc for good measure) and you have a pretty standard helo package for us. This could then downsize to 2 NH90s and a A109 once it all settled down if need be (though probably not as likely for redundancy reasons). Aus army also had 3 blackhawks supporting their battalion in timor leste post 06 ops so obviously a considered option.
That would leave 5 NH90s and 4 A109 in NZ for tasking, training, rotation and maintainence keeping in mind that with main effort being a Bn gp overseas, not much else commital wise will be happening around this time due to manpower required to sustain, raise, retrain such a large deployment (for NZ). So realistically the most NH90s we would deploy at once should be 3 at the most, 2 for coy sized ops and 1 for smaller tasks along with the obvious A109 option to supplement or even conduct said task (unlike the sioux of old).
I don't see us getting any 'extra' 90s just yet which is why I think a way to alleviate any perceived 'shortage' of usable frames is to just replace the seasprites with a mix of NFH90 and marinised 90s when the time comes (obviously coinciding with naval ship replacements as well, namely ANZACs and otagos). Give 6 squadron say 4 NFHs, 3 standard 90s and marinise 2 of the extra 3 109s and there are your extra NH90s (for a total of 11 baseline NH90s). Makes sense as no doubt a majority user will be this JATF we keep hearing about operating from naval vessels so why not keep them with the SME squadron. 3 standard 90s under navy control just gives govt another option for taskings such as TC Winston, DOC island tasks, organic JATF flight etc and cuts down on required currency training keeping the likes of 3 sqaudron fully versed in maritime ops as working from sea is completely different to merely being transported via sea, a key difference between 6sqn and 3sqn. This will also open up more tasks to our naval crews with a more optioned fleet that would otherwise be the domain of their air force counterparts, sharing the load so to speak.