How many players were trapped inside the game in the original Sword Art Online story arc?

How to get close to this number

Reason from the premise. The story needs a population large enough to form a society with guilds, economies and factions, and small enough that named characters can plausibly encounter each other repeatedly. Ten thousand is the round number that satisfies both, and round numbers are the norm in fiction.

Contrast that with the Squid Game figure elsewhere on this site, which is derived from an in-world calculation. Fictional counts split into two kinds: round numbers chosen for scale, and specific numbers derived from a rule in the story. Recognising which kind you are looking at is most of answering a fictional-count question.

The premise itself became genre-defining, which is the part worth knowing: a virtual world with the exit removed and real death as the stake. An enormous number of subsequent works are built on the same setup, so a figure from one 2009 light novel describes the founding text of an entire subgenre rather than an isolated story.