How many people personally known to Winston had disappeared at one time or another?

thinks
about people disappearing without a trace and ruminates that

Perhaps thirty people personally known to Winston, not counting his parents, had
disappeared at one time or another.

Later in the text,
Syme, the language specialist, will disappear without a trace.

One of the
most unsettling parts of life in under the Party is the way a person can simply vanish one day
and be treated as if her or she never existed. The threat of being disappeared is an important
way the state is able to terrorize people and keep them in line. As Winston notes early in the
novel, in a country without laws, any minor transgression can become a crime. People's fear of
being scrubbed from history helps make them very fearful of doing anything that could be
considered unorthodox. In a darkly comic moment, even the idiotic Parsons end up in prison with
Winston, having been denounced by his daughter.