I think 16 is acceptable if you're not running brainstorm. So I would say that 12+4BS is not as good as 16 non-BS cards. This comes from my testing of Counter-Goblins. Consider a hand where you have Force + Brainstorm in hand with no other blue cards. Often when running only 12 other blue spells spending that BS to find a blue card is a huge gamble. In a deck with 17 or 18 blue cards, casting the BS will more reliably find you a blue card.
Also fetchlands can play into this as well, but in a more minor way. When I was playing the counter-gobs list I had 4 FOW, 4 Daze, 3 Curiosity, AR, TW, twister, and 2 Razorfin Hunter. Thats 16. In addition to that I was running 6 fetchlands. I would say that FoW was generally supported, although there were a few times I had to pitch Timewalk or Twister ... that was painful, but ultimately for the better.
Its also good to point out that if your on the edge at 16 or 17, be careful how you board. If you're bording in alot of non-blue cards you might consider cutting back on the forces as well.
I've played hide seek fish without brainstorm on 16 cards. I've always felt safer in the 18-19 range, but it's most certainly possible, like said above, to support 4 force of wills, and 12 other blue cards. You'll have random times where you will not have a second blue card, but thats what happens when the blue count goes below the safety range.