@isekaiweeb The number of orcs seems to be a translation issue repeatedly. Before the battle, they were going back and forth between 20,000 and 200,000, after they sustained some casualties it began alternating between 15,000 and 150,000. However, the population of the city is likely in the right ballpark.
Recall that Rimuru suggested the orcs should spread their people out to work in places all over the forest, so regardless of which numbers were right (20,000 before and 15,000 after battle if I had to guess) it's not like the entire orc population is in his place.