I first review the due cards. For those very card, I would certainly love to use varied practice and see every cards at random. In order to do this, I create a filtered deck, which shows in random order the due cards. The filter is simply «is:due».
Once due cards are done, I move to new cards. Here is the beginning of the reason I need a lot of decks. I know that I can see 200 new cards by day without too much trouble. This is indeed what I have done for months. However, I certainly can NOT see 200 new cards which all are related to the same subject. I need new cards to come from different decks for two reasons:
- 200 cards related to tea, for example would be far too much. While knowledge about tea is not extremly difficult, it would just create confusion in my head, and impair my learning.
- most of my notes makes reference to notes seem before. If one cards contains a question such as «what is the area of a circle», I don't want to see this question before I've seen the cards defining «circle», «radius», and «area» before. I don't want to see all of those cards the same day. Thus, I'll limit the number of new card in this geometry deck to one by day.
But, while I take my time to learn geometry, I can certainly learn new thing in group theory, in analysis, in python, in automata theory, and in a hundred of other subjects (literally). In order to let Anki knows which card belong to which subject, whether a card is physics, analysis, group theory, ring theory, guitar or cooking, I have to use decks. Using tags would let me know which is which, but would not allow me to precisely ask for one/two card of each subject every day.
In order to give a more concrete example, here is the list of decks which are not subdecks: Algorithm, art, chemistry, computer science, economics, people, hardware, language, logic, mathematics, other, physics, programming, software, everyday life. In Mathematics, the subdecks are Algebra, analysis, category, combinatorics, numbers, geometry, information theory, measure theory, number theory, probability, topology. In the Algebra subdeck, there are: linear algebra, semigroup theory, abstract algebra (Summit-Foote). In this last subsubdeck, there is a subsubsubdeck by part. Group, field, module theory, representation theory, and rings.
 With a few exception. For example I began recently a guitar deck. I certainly want to see this deck only when I do have my guitar with me, and not when I use anki in the subway
 You can download a copy of my profile, minus some personal cards I do not want to share, here. It weight 3,7MO and contains LaTeX code for more than 50 000 formulas. If you try to «check media», it will be VERY LONG !
 And commutative algebra, but there is no new cards, since I lack a lot of definition from the previous parts.