"Castellano" is the official language, which is taught, in School, or Language academies (it is availed, but the Royal Spanish Academy) which is the institution which regulates the spanish language, Grammar and syntax for Spain and all the spanish speaking countrys.
In US and UK it is called the Neutral spanish which is used in legal, conference, and comercial comunications.
Spanish dialects, are well easy about 50 +( In spain there are alone like 11 or 12 dialects or more Trope shure can provide serious numbers).
For example, Mexicans tend to say to the color brown (color café) which is not used or rarely used by the rest of Spanish speaking countrys, they use (marrón).