Original Fallout lead Tim Cain says the team planned for there to be 1000 Vaults, but he thinks there are actually far fewer in the setting

pcgamer.com:

In a new vlog on his YouTube channel, veteran programmer and RPG designer Tim Cain revealed some more of the background he and his team initially planned for Fallout's vaults. The crew at Interplay conceived of there being 1,000 Vault Tec vaults in the United States, a figure with some in-game and real-life logistical issues that Cain then explores, all with the caveat that this is him having some fun: «This is just me talking about it⁠—it's not canon!»

First and foremost, the 1,000 number was just an aesthetic choice: «The only reason this pin was even placed in the original game was when Leonard [Boyarsky, Fallout 1's lead artist] was doing the vault suit, he asked me how many digits he'd need to keep on the back.» Cain said. «I thought, well, let's just say there's a round number, there's a thousand.

»I'm a programmer, I start counting at zero, three digits."

The idea of there being a «Vault Zero» then figured into a Fallout lore tidbit Cain previously shared on his YouTube channel: the vault experiment was in preparation for long haul interstellar travel. I first thought of the LA test vault from Fallout 1 as a Vault Zero candidate, but apparently there's an explicit "Vault Zero" in Fallout Tactics.

Cain also casually dropped that another Fallout lore keystone was borne of stylistic concerns: the thirteen «commonwealths» of Fallout's crueler, more brutal America only came after Fallout's artists designed a thirteen-star American flag for the game. «According to Leonard, [the Fallout art team] just did it because it looked cool.»

But even with 1,000 vaults in the US, Vault Tec wouldn't have saved that many people, maybe «a third of 1%» of the US population by Cain's reckoning. «If each of these vaults held something like a thousand people, that's only a million people that are going to be saved in Vault Tec vaults, which is way less than the population we imagined.»

Also, while artistic license and fudged scale is definitely at play in these games, a thousand