a number: the total reached by counting, whether the size of a group or a specific figure that stands for something.
The core meaning is plain: a number, the total you get when you count. Most of the time the word names the size of a group, how many people or things there are, and the New Testament reaches for it when a crowd of believers is being tallied or a fixed figure is being reported.
A couple of uses stretch that core without leaving it. In one line a person is described as being "of the number of" a group, which simply means he is counted as one of its members. And in Revelation the word names a symbolic figure, "the number of the beast" or "the number of his name," where a number stands in for a person's identity rather than measuring a crowd.
All of these still turn on the same idea of a count or a figure. Where a writer wants to stress a large throng rather than an exact tally, the New Testament uses a different word (πλῆθος).