A segment decides who a campaign reaches.
Conditions, not a list
A segment never holds names. It is a set of conditions, re-run every time it is used.
That is what keeps segments current: someone who starts matching today is in the segment tomorrow with no work from you, and someone who stops matching drops out.
If you need to send to a fixed, frozen list, a segment is not the tool for it.
The number you see is the number that sends
The count beside each segment is not stored in advance. It is worked out then and there, by the same engine a real send uses.
So it is not a second, separate estimate that can drift from reality. It can only be a few seconds out of date.
Three sources, chosen separately
Conditions can cover customers, contacts and leads, and each is selected independently.
If the count is lower than you expected, check which of those three are actually included before reviewing the conditions.
A segment does not override unsubscribes
Anyone on the suppression list is left out of a send even if they match every condition in the segment. That is why the number actually sent can be lower than the segment's own count.