Raen'Vine's not sure exactly how many humans there are. Maybe a million, but he could be off by an order of magnitude for all he knows. There's a small city in/near each Grove, and a few smaller outskirt settlements heading farther away.
Dryads have a few options, though none are great. They grow constantly, and their minds and memories are decentralized throughout their entire bodies. If they don't want to plant themselves, they can cut themselves in half and become two smaller dryads with half of the original's mind each. They can also cut off pieces and send them to be grafted to a grove. They have some control over where each memory is, but it isn't perfect. Oh, and at least one dryad has decided to turn an old boat into a chassis and now goes up and down one of the larger rivers.
Merfolk have wings like flying fish or manta rays or penguins, or extra and more elaborate fins. Even if they were given flying wings, there's only so long they could stay outside of water without suffocating. Raen'Vine doesn't know of any fishfolk demigods - the gods really hated them up until a few centuries ago and have been distant since. He's also never seen a dryad demigod, or any plant monsters in general.
There does seem to be a limited amount of each magic being put into the world at a time. This comes up in every system he knows about. Vampires only rise if there aren't too many vampires around, pegasus won't breed until the total population is low enough no matter how far they get from the others. There is only so much orichalcum, and only so many other artifacts lying around. Potions take special liquid as a base, which comes slowly from certain fountains. There can be - could have been, he corrects himself, as wizardry seems to have vanished - as many wizards as they want to share their power with but the more wizards there were the slower each one would regain mana. That meant that sometimes there would be a few very powerful wizards, and sometimes many who could barely light candles.
One magic system he vaguely remembers from long ago had also vanished. It involved wrapping up poppets of herbs with a single human hair, then throwing it into a fire to cause effects to that person. Different combinations of herbs did different things. When the herbs were rare and gathered naturally, the magic worked consistently. When they began being farmed, the magic was unreliable, working only so many times a day at random and often doing nothing at all. People would forget, and after a time it would be rediscovered, to repeat the process over and over. He assumes the god stopped... doing whatever it was they did to make magic work, at some point, but he's also never bothered testing it. Most magics that increase or decrease someone's strength or speed or such usually make him nauseous when combined with his own power.
Mages - there are only so many mage-hearts that can transform people into mages. Each will regenerate 5 to 10 hours of magic in a week. He gets 6, himself. He's not sure what causes the slight variation there. They can store as much as they want in chargestones, and to an extent overdraw some extra if they want to deal with some unpleasant side effects.