Numbers 16:15

"And Moses was very wroth, and said unto Jehovah, Respect not thou their offering: I have not taken one ass from them, neither have I hurt one of them. And Moses said unto Korah, Be thou and all thy company before Jehovah, thou, and they, and Aaron, tomorrow: and take ye every man his censer, and put incense upon them, and bring ye before Jehovah every man his censer, two hundred and fifty censers; thou also, and Aaron, each his censer. And they took every man his censer, and put fire in them, and laid incense thereon, and stood at the door of the tent of meeting with Moses and Aaron. And Korah assembled all the congregation against them unto the door of the tent of meeting: and the glory of Jehovah appeared unto all the congregation."

Note that Korah is the one who assembled the people, his purpose, no doubt, being that of demonstrating the great "victory" he had won before Moses. Lo, Korah himself and all of his company will go right in and perform the highest function of the priesthood, and everybody will see it! Well, that is, no doubt, what he thought.

Regarding the 250 censers: There were definitely more than that, because after that number was given, Korah also and Aaron, were also designated to appear with their censers (Numbers 16:16). The number 250 is therefore a round number.

Then what happened?