How many fish should you have in a tank is always a question that gets asked a lot. Here is food for thought and things to consider when stalking the tank. Remember there is no "magic number" as each tank setup is different, stocked with different creatures, and provides different homes based off the aquascape.
1) Remember that your local fish store (LFS) has "Holding tanks". They have a lot of fish in each tank as they are trying to sell them off before the bio load starts to kill them off. Yes some LFS will have display tanks and move fish around according to help spread the bio loads they hold, but generally speaking they buy what they know will sell with in a week or two so they can order more fish. The number of fish in a LFS tanks is a not a reasonable expectation for long term inhabitants of our home aquariums. Lots of health problems, bio loads, and stress will occur and deaths will start to manifest.
2) Bio load.. What is this? Will its the amount of food waste and fish waste (poop) is generated in a given time period, and what the tank has to handle this. Skimmers, Filtration (biological and mechanical), Micro Algae, all play a role in this. When people talk about tank age, a lot of times this is in reference to biological filtration of how established bacteria is in the system to break down all the bad stuff, general rule of thumb older the better. Just know that when you add in a pet aquatic friend you increase the bio load, and it will take time for the tank to adjust to the new load. Eventually you will hit a cap where you generate ammonia faster then your tank can process it nuking the tank, or you will have so much nitrogen in the water it slowly poison all of your friends.
3) Stress.. To many people in a small places causes stress, this is even true with fish and other creatures we place in our tanks. Top it off reefs are not "peaceful" places to live someone is always fighting someone or trying to clam a home on the rocks and in the caves or some corner. We do our best to make sure all of the creatures we put in are "peaceful" when put together, but if there is not enough room for all the creatures to have room they will fight.
My suggestion is look what fish you want and research to figure out where they live or spend most of there time. Are they bottom dwellers, middle, top? what type of food do they go after? is there enough to sustain them and others? Are there spots in the tank that have not been "claimed"? If these are yes, then you can probably add another fish if your your setup can handle the bio load
I always recommend adding fish slowly, One or two here a couple of months later one or two more and watch your parameters.. Mainly those in the nitrogen cycle, if they stable out and are manageable then you can try adding another fish if there is "unclaimed space" for the area the fish will want to live. Just know territorial issues can and will arise.. prime example: Clowns and Damsel fish are very very territorial and always try to tank up the whole tank. I had a pair of clowns that ran my 40g gallon tank and would try to kill anything that went in. So go slow and watch, give it a few days and if fins are being eaten separate the fish.
Just my two cents and how I approach stocking my tanks
