Freshly hatched baby brine shrimp is sufficient to sustain them, but it's been shown that enrichments high in HUFA works wonders in prolonging their life and promoting breeding.
The amount you feed depend on volume of water, not on the number of seahorses. Seahorses are opportunistic ambush hunters. They wait for the food to "pass by their doorstep". You should maintain a baby brine shrimp density of a "light flurries" in Canadian terminology
.
I feed my 40gal breeder with 200 dwarves 1/8 teaspoon, hatched out at assumingly 75%, per day.