The number sign # sometimes used in mathematics to indicate the number of a quantity satisfying some condition, e.g., 
. The symbol is also used to denote a primorial.
Octothorpe
See also
NumberExplore with Wolfram|Alpha
References
Bringhurst, R. The Elements of Typographic Style, 2nd ed. Point Roberts, WA: Hartley and Marks, p. 282, 1997.Referenced on Wolfram|Alpha
OctothorpeCite this as:
Weisstein, Eric W. "Octothorpe." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Octothorpe.html