Instant Ocean makes a good one that reads all the way down to 1.000 (complete freshwater) which is necessary when dealing with such low level brackish as bbgs need.
You could do 3-5, but I would not do more than that. They are small and bioload-wise you could do more, but they are also somewhat territorial and need space to fan out and have "caves" and plants to breakup the line of sight.
Here's a link to a good rundown of how to mix brackish water and specifically how to raise your salinity. You should not raise the salinity by more than 0.002 per week so as not to kill the bacteria before it can acclimate. Most of the time, brackish fish are sold as freshwater and need to either be started in a cycled freshwater tank and slowly raised to the point they need to be at OR you can cycle a brackish tank at the level you want and then just acclimate the fish by drip method over the course of a couple hours.
Here's a link to an article about salt, specifically the differences between aquarium salt, marine salt, table salt, etc.
You could do 3-5, but I would not do more than that. They are small and bioload-wise you could do more, but they are also somewhat territorial and need space to fan out and have "caves" and plants to breakup the line of sight.
Here's a link to a good rundown of how to mix brackish water and specifically how to raise your salinity. You should not raise the salinity by more than 0.002 per week so as not to kill the bacteria before it can acclimate. Most of the time, brackish fish are sold as freshwater and need to either be started in a cycled freshwater tank and slowly raised to the point they need to be at OR you can cycle a brackish tank at the level you want and then just acclimate the fish by drip method over the course of a couple hours.
Here's a link to an article about salt, specifically the differences between aquarium salt, marine salt, table salt, etc.