You may have better luck posting this question in /r/genealogy, but that number of descendants doesn't seem extraordinarily small by any means. The Disney last name is actually a very old Norman last name, brought from France to the British Isles after the Norman Invasion in 1066 when fixed permanent surnames first began to appear. (A subject which I have written about in this sub before here.)

Disney derives from the town Isigny-sur-mer in Normandy, so the original derivation was d'Isigny (someone who was from Isigny). As these things go, there would have been no fixed spelling until the family themselves started to write it down when they became literate, probably many centuries later, and they decided on the Disney spelling because, phonetically, that's how it sounded to the British- or Irish-born ancestors. But also as these things go, there may have been multiple different spellings of the last name coming from different ancestors of the original Norman progenitor, as different branches of the family became literate at different times in different places and decided on different spellings. And then later on down the line, there may have been other ancestors who decided the change the spelling, so there could be several variations on the "Disney" surname taking that ~5000 number to something somewhat larger.

My expertise is more in New York history, where in the early years, a lot of new surnames arose because the early Dutch didn't have fixed surnames. When the English took over the colony, they forced the Dutch population to stop using Dutch patronymics and start using English fixed surnames.

Out of this era, the most successful surname is often said to be Voorhees which itself has several variations such as "Van Voorhees", "Voorhies", and "Voorhis". In total, there's something like 10,000 living people with one variation or other of the surname.

Less successful is a surname like "Roosevelt" which also came out of that era and they have somewhere around 5000 living family members with the last name.

Both surnames can be traced back to a single immigrant ancestor (though there were probably a few unrelated people who changed their name to "Roosevelt" in the 1900s after one of the two Roosevelt presidents), so we have a pretty good idea of the success of these surnames.

Even less successful are the surnames Van Nostrand/Nostrand and Conselyea, who both have streets named after them in Brooklyn, but who each have less than a thousand living descendants, even though they were both Dutch families who arrived in New York around the same time as the Roosevelt and Voorhees families did.

The Disney surname is much older but even so, it's not particularly out of the ordinary for the family to number ~5000 people worldwide, particularly because the evidence points to the surname all being traced back to a single Norman ancestor (though, like with Roosevelt, there may be later adopters who took the name in honor of Walt Disney).

Contrast this with surnames like Smith, Carter, and Johnson, where there wasn't just a single ancestor who introduced the name, but many different families from different parts of the British Isles as well as other parts of Europe who anglicized their similar names when immigrating to America, and there can be hundreds of thousands of people with those surnames. Different unrelated people were called Johnson, and just because your last name is Johnson, you aren't necessarily related to some other person named Johnson--well, at least, not through the last name. (Everybody is related to everybody eventually, after all.) But with a name like Disney or Roosevelt, aside from people who changed their name to honor the famous name-holders, you are likely related to all other living Disneys or Roosevelts.

In other words, there may be a thousand progenitors of Johnson who have 5000 ancestors each, bringing the total number of Johnsons worldwide to five million people. But with Disney or Roosevelt, there was only a single progenitor and, thus, the total number of living family members is only 5000.

So Disney is somewhat of an uncommon name, but uncommon names are pretty common, and have varying levels of success. Some have died off entirely, some only have a few hundred descendants left, and some may have tens of thousands, but few such names ever reach the hundred thousands or millions. There are many, many names out there, even old ones, that remain in the low thousands because they come from a single point of origin. Those more successful surnames like Smith or Jones usually have many, many different points of origin.