It sounds like you have an excellent set up for seahorses. An aquarium of 40 gallons (150 L) can provide significant benefits in terms of the greater stability a larger volume of water can provide. With the added water volume of the sump, an aquarium of that size will be more resistant to overcrowding and to rapid fluctuations in temperature, pH, and salinity than smaller setups. The larger the aquarium the larger the margin for error it offers the aquarist and the greater the benefits it provides in terms of stability. So I40-gallon aquarium with lots of live rock, an efficient protein skimmer, and the added advantage of a sump should make an excellent homes for your seahorses. As experienced marine aquarists and reefkeepers, you have the right background to become excellent seahorse keepers!
Assuming that your aquarium will be a dedicated seahorse tank and not a community tank, the suggested stocking density for Mustangs and Sunbursts (Hippocampus erectus) under those circumstances is about one pair per 10 gallons of water volume. So a reasonable number of average size Mustangs (or Sunbursts) to keep in a 40-gallon aquarium with a sump is a total of about four pairs or eight individuals.
However, since you are new to seahorse keeping, it’s a good idea for you to keep your aquarium understocked while you’re learning the ropes and gaining some valuable firsthand experience with these amazing animals. And it’s always a good idea to increase the bioload of your aquarium gradually, rather than stocking your aquarium to capacity in one fell swoop, so one pair of Mustangs plus one pair of Sunbursts should make a very good starting point for you, and your aquarium is more than spacious enough to accommodate them.
Best wishes with all of your fishes! Good luck with your new seahorse setup!