If you are buying brushes to add to an existing collection, or buying on another person's recommendation, it is important to appreciate that a particular size designation doesn't translate into the same physical brush dimensions across different manufacturers' ranges.
There is no industry standard that manufacturers in different countries and brush-making traditions adhere to. Today's increased choice through international trade and the rise of mail-order shopping make this more of a potential pitfall than a few decades ago.
Round pointed brushes tend to be fairly standardised in the smaller sizes, but can diverge markedly further up the scale. Riggers are even more variable.
If someone tells you to buy, for example, a #3 rigger without also telling you the brand or the tuft dimensions, the size alone cannot safely be used to make the correct purchase.