Me being me, I do think that the NZDF needs to work out just how many naval helicopters they want to have available in a decade, and then work from there to determine how many are realistically required to achieve that number, whilst also keeping in mind what the naval component of the NZDF is likely to look like towards the end of the 2030's and into the 2040's.
As has been mentioned, the NZDF currently has five naval helicopters, but what gets left out is that a further three were also purchased and have been getting used as a source for spares. Should NZG be cheap again, (yes I mean cheap, not frugal) five platforms might be adequate initially, but as they clock flight hours and/or parts start getting harder to obtain that five might easily dwindle first down to four, then three, and perhaps even only one or two actually in service. Anyone care to see a repeat of the C-130H issues where NZG called upon the NZDF and RNZAF to carry out an op, only for it to be forced to be scrubbed because the only platform available had an error/failure and had to abort?
Or heck, just look at NZ's initial SH-2G(NZ) acquisition which was also for five platforms, and then the problems which NZ encountered with keeping them available and in service due to the small number of platforms. Such issues were also why when NZ retired the SH-2G(NZ) and replaced them with ex-RAN SH-2G(A) modified to the SH-2G(I) standard, ten platforms were purchased with eight to be available for service whilst two were to serve as a source of spares.
Now there is also the larger question of what the size and composition of the RNZN fleet will be going into the future, as well as what type vessels should either have an embarked naval helicopter, or be kitted out so that one could be useful at least part of the time. IMO having a naval helicopter embarked aboard an OPV or other vessel with a comparable sensor, comms and electronics fitout would be a waste because they would have neither the electronics to benefit from any increased situational awareness from a naval helicopter aloft, or have armament to contribute to any fight an embarked helicopter might engage in.