Even more Daihatsus? How many should I have (maybe 6, two per expedition fleet)?
I would argue that you want to have more than that.
If you don't have 2-3 Type 89 Medium Tank & Landing Force, then you are going to want to use 2-3 Daihatsus to upgrade to those.
If you don't have 2-3 KaMi Amphibious Tanks, then you are going to want to use 1-2 Daihatsus to upgrade to those (there is one tank available via a quest-chain)
Both of these are important for anti-installation purposes.
For resource-gain, DevSquad74 is spot on, but Toku Daihatsu are better. Unfortunately, these can only be made through Akashi and cost 4 Daihatsu to convert from a +10 Daihatsu to a Toku Daihatsu.
So, for strong resource gain, you are going to want 4 Toku Daihatsu per fleet.
Just for number-crunching sake, you are going to want the following amounts of Daihatsu:
- 3-5 Daihatsu to upgrade for the Anti-Installation gears (if you don't already have them)
- 11 Daihatsu for your expedition fleet (12 if you do not have Kinu Kai Ni).
Then, for each Daihatsu you turn into a Toku Daihatsu, you will need to spend 4 more Daihatsu, which means that you are going to want to have total of 20 Daihatsu on-hand (or over time) per fleet (1 to upgrade + 4 to convert = 5 x 4 per fleet = 20), for a grand total of 60 Daihatsu (assuming you don't have Kinu K2).
...and then, assuming you think you're done, there is upgrading each Toku Daihatsu to +10, it costs a Daihatsu to upgrade each point pst +6, meaning that to reach +10, you will need to use another 4 Daihatsu.
...so yeah, if you are going down the rabbit hole of maximum resource gain per expedition, there is no such thing as 'too many Daihatsus', and every single one of these things is a more than welcome addition.