Large forest that are as large as half a continent, like the rainforest, maybe..Also, large forests easily support waaaay more than 150k creatures.
Considering that the orcs traveled here on foot in mere days and the overall distance between cities we've seen so far however... no, no way.
How far was Dwagon away again? 3 days?
Well, as has been said, it's clearly not the only thing that made little sense... but it's strikes me as immature worldbuilding when they bring these numbers in like it's nothing special.
What I'm saying is.. I'm pretty sure this show would've been just as enjoyable, maybe even more so, with less inflated numbers. Even if an enemy in the future has a huge army, "huge" is still a relative term... meaning if the author wants to outnumber Rimuru's forces again, he has to go full USA level military again and all of a sudden, you are in the millions.
And when you are in the millions, it's even more annoying to see them perish with a snap of a finger (which already happened to the orcs)