Bear's Number is an enormous upper bound to a certain problem in Multiversal Ramsey Theory. It is the minimum number of bears such that 5 identical co-planar bears is forced regardless of the space/manifold/structure containing them. The number of required bears for forcing is known as B*, and a mysterious figure by the name of Robert Bear is claimed to have written a paper (that can't be found fAsome reason) that proves that B* <= Bear's Number. A common joke however goes that B* may actually only be 5. Clearly there is room for improvement ... so much so in fact that even Ronald Graham used to roll his eyes at this!
Lower Bound
It is trivially the case that B* >= 5. This is because you would need at very minimum 5 bears to have 5 identical coplanar Bears. The actual size is unknown. Some argue that it either can't be known or should never be known, because the 5 bears would form a pentagram in all verses that would unleash the forces of darkness and end existence. There are some that argue this has already happened an existence has already been replaced by something even more inexplicable (hence our general confusion). Most however think all of this is a load of dingo's kidneys.
In any case, as a kind of warm up to the actual problem it is worth noting that simply having the minimum required number of bears is not necessarily sufficient for forcing. Consider the case of whether 4 bears is enough to force 4 identical coplanar bears. It can be shown without too much difficultly that there exists a space in which 4 bears can exist and not all be coplanar at once. If we take our space to be normal 3-dimensional Euclidean Space and we assume the set of Bears to at least contain a single element, and we assume for the sake of simplicity that bears are point-like, then we can arrange this point-like bear in a tetrahedral pattern in which each vertex is a bear. For every set of 3 bears they form a plane of which the 4th is not a part of. Hence you must need more than 4 bears to force 4 identical coplanar bears in all space/manifold/structures.
Upper Bound
There is no known upperbound to Bear's Number, because we do not know what the totality of all spaces/manifolds/structures is, nor the totality of all bears, nor the intrinsic properties of all bears. Bear's Number mysteriously first appeared in NO! video 0 to ATE, occurring after Absolute Everything. It can be surmised that it's placement in NO!'s list is his best estimate of it's size. It occurs directly after U^^^I and before Superfinity. It is not known whether this is merely a guess, a proposed conjecture, an upperbound, or if NO! is in position of a proof either proving this placement or demonstrating Bear's number must be less than Superfinity. Any speculation on the matter is just speculation. Ask NO! about it sometime.
(Possibly Apocryphal) Origin Story[]
There is a rumor that the first person who conceived of Bear's Number was a humble lumberjack by the name of Robert Bear. Bear had no formal training in mathematics up to this point and no interest in it. Then one day when out on a lone hunting trip in bear country, he tripped over a bear skull and knocked himself senseless. When he came to he had a miraculous vision of Bear's Number in the sky forming a Pentagram of transomniversality. He went temporarily insane from the size of the number. Later when he came back to his senses he had apparently been endowed with superhuman mathematical abilities. He sat down and wrote a paper in which he apparently proved that Bear's Number was larger than the sum total of absolutely everything, that is to say, that the forcing never actually occurs. To make the forcing happen anyway he built a strange esoteric system whereby it was forced under some size beyond Absolutely Everything but just before reaching a number he called Superfinity. The paper was centuries or even millennia ahead of it's time. The consequences of his paper to the nature of existence are profound. AD Wiki should probably look into this!
Anyway, he then went on to submit his paper "On Bear's Number and the confirmation of sizes extending beyond the absolutely endless totalities of all existences" to the Very Real Committee of Very Real Mathematicians. It was thrown into a wastebasket immediately upon submission, not because it completely flew in the face of the mathematical establishment or because it's a fundamentally nonsensical self-contradictory pile of rubbish, but because it was published in green ink, which was against their submission policies.
It is not known what happened to the paper or Robert Bear after that. Some say Robert Bear ascended into the realm of the non-existent. Other's say he never existed at all and all this is totally made-up garbage. One thing is known for sure though. No one knows anything about the whereabouts of Robert Bear. The paper itself also seems to have vanished from existence or never existed. Perhaps both Robert Bear and his Mysterious paper were zapped out of existence by some deity to keep humanity from learning too much. Who knows?
In any case, it is supposed that NO! somehow discovered either the paper or learned of it's existence and thus decided to add it to his number list.
Demonin's Conjecture[]

Demonin's Conjecture
Demonin has conjectured that it's actual value of B* may only be 5. The upshot of this would be, that no matter what reality you found yourself in, if there were 5 bears they would all be completely indistinguishable and there would exist a plane that they would all lie on.
Although it is often assumed to necessarily be finite because "number" is in it's name, this is not the case. Some cardinal numbers are infinite despite being numbers.
Trivia[]
It is also a mysterious number which appears after both Absolute Infinity and Absolute Everything in the NO!'s video 0 to Absolute True End. It is past THE OTHER SIDE, BEYOND THE NUMBERS, and even THE END.[1]
𝙉𝙐𝙈𝘽𝙀𝙍𝙎 0 𝙩𝙤 E??̸͓͌A̵̰̘͙̿̈́͝И̷͙͇̒?̵͖̦̘̉͆̌Ⴇ̵̻̪͗́̉͐͂Á̴͚̋͝⅃A̷̛ͅƧ̴̡̥̪͍̮̉͂̿?-̷2̷͚̹͉͂̅-̷?!̵͙͓̣̰͗!̷!‼‼‼‼‼‼
Yes.