Within the same series, you can assume that cards of the same rarities are equally numbered. However, across different series, the number of copies of each rarity will often differ. Be sure to take into account when trading. (More on rarities here.)
As an example, the following trade shows a trade with two extremely rare cards. Although both cards are extremely rare, these cards are not equally scarce because 200 copies of "Air Joe" exist while there are 1200 copies of "KEV-19."
Remember, some cards can have thousands of copies, while others have just a few hundred or even just a few dozen. Some cards have no print limit whosoever (this occurs for "unlimited" series). The lesson is, be careful assuming that all cards of the same rarity are equally scarce or valuable!