What are the different rarities of tokens in Magic: The Gathering?
I couldn't find an official answer, but I can make an educated guess based on how cards are produced in general.

Cards are printed in big sheets and then cut out and randomized. These sheets are grouped by rarity. You print sheets based on the desired ratio of rarities in the final product, e.g. 1 rare sheet per 10 common sheets if the two sheets are of equal size.

Sometimes, more than one copy of a cards is on each sheet. For instance, early sets, particularly the small sets which tried to simulate three rarity levels with only two print sheets, had multiple copies of some cards -- this is why Camel is listed as "Common 5" in some collector guides, or why Knights of Thorn is an "Uncommon 1" (functionally a rare for The Dark). For a more modern example, consider mythic rares: mythics don't get their own sheet; instead the rares sheet has two copies of every vanilla rare and one of each mythic.

This is undoubtably how the ads and tokens sheet is arranged as well: all of the ad and token cards are printed on one sheet, but there are multiple copies of the "common" tokens and few copies of the "rare" ones. For example, a whole sheet of around a hundred M13 ads and tokens might contain twenty goblins and only one Liliana emblem. The exact mix of cards is likely to vary from set to set, but you can get a good idea of the rarity of "rare" tokens if you know the overall sheet size.