It's always the adult fish you take into account when working out stocking. Juvenile fish, which is what are in the shops, grow very quickly.
There are so many things to consider when stocking a tank. Some people swear by the 1 inch of fish per US gallon and can get really nasty towards those who exceed it. Others insist on the 1 inch of fish per 12 square inches of water surface. More people insist that both are old fashioned and should be ignored.
I do advise newcomers to the hobby to stop slightly short of what the community creator says they can have, simply because there is a tendency to go overboard. And the newcomer will not have a mature tank (as opposed to a cycled tank) and may not have got into the maintenance routine yet that is needed for a tank stocked to something approaching its limit. Once the newcomer has more experience, then yes stock up to the CC limit, or even beyond. By that time, the fishkeeper will know his tank and his fish. He will know when something is not right. And he will now know that if he wants a lot of fish, he needs to do the maintenance necessary for the the number of fish in the tank.
And yes, aqadvisor is not well regarded. Some forums go as far as to ban the name, treating it the same as a swear word that is removed by a filter.