Alright. If there are two billion lions, then it means, there are 20 to 200 billion gazelles, right? Lions need food. Pokemon are not a lion's food, since we need them to find out who wins.
For 200 billion gazelles, we need a lot of food for them, too. So it means, a lot of space for grass to spread.
This means a much bigger planet than the earth. I mean, you can say that the pokemon planet is much bigger than earth, but that means higher gravity. But the size of a planet and the strength of gravity is not linear, in fact it's logarithmic. That means, to have space for all the lions, all the gazelles, all the grass and of course all the pokemons we need higher gravity. Lions are built to handle 1g on earth but not 3-4g.
tl;dr too many lions, all of them will die.
