It will cost us more if we get block1/2 then upgrade it later to block 3, moreover there will some parts that will be replaced and thrown out such as the semi recessed launcher, external targeting pod etc. Getting block 3 won't waste extra money but will need to wait years.
Thus ordering 16 unit for now is a good choice, its just the right number to balance our urgent need of new fighter but also not wasting extra money for the upgrade.
The delivery of the 16 unit will take several years and after all being delivered with waiting a few more years hopefully the block 3 already entering production. By then we can order 32 unit of block 3. Thus its for long term acquisition planning.
Ordering for just only 16 unit overall is meh. Thus later should order 32 more.
Total 48 as our previous commitment, and IMO no need for more than 48.