Uhh... the classroom I work in has at least 16 (bolds and pastels). I can't imagine even if chalk afforded more colors that they would be distinguishable enough to be useful in technical contexts.
Oh I don't doubt that humans can distinguish between more than 16 colors. But white/blackboards are used mostly for sciences and business, not drawing. Trying to distinguish between turquoise and aqua in a graph or formula is difficult at best. (And I really wish infographic/map designers understood this concept.)
Uhh... the classroom I work in has at least 16 (bolds and pastels). I can't imagine even if chalk afforded more colors that they would be distinguishable enough to be useful in technical contexts.