And just to tweak this a bit...
The number of rooms is less than 30,000. Closer to 27,000. And it is rare to see 100% occupancy. 80%-85% is considered to be great in the hospitality industry, though Disney usually hits that mark. Sometimes even a bit more. 87%-92% is often achieved. So the number of Disney guests is even a hair lower than the quoted estimate. As noted earlier, 75,000 on site guests is a good estimate.
But not all of those people are in the parks. Some are just arriving for the day, using up a room for the night without going to a park. And the people who checked out of that room might have headed to the airport or driven home without going to a park that day. Other guests are attending conferences. Others are at Universal. Others are taking a "rest day" at the resort. So even if Disney resorts are accommodating 75,000 people per night, maybe 60-65,000 of them are going to parks. Now put 45,000 people in the MK, 40,000 in Epcot, 35,000 in DHS and another 35,000 in AK and you get 65,000 out of 155,000 as on site guests. And as park attendance rises beyond these levels the on site percentage goes down as the added attendance has to come from off site if we have assumed 90+% occupancy of Disney hotels. So on a really crowded day it might be 65,000 out of 200,000.