Numeral cards, an inner numerology
There's a much overlooked fact about the numeral (or pip) cards that makes a strict numerological approach fails: tarot pip cards express a different actualization or materialization of different numbers.
Note that in the following short text, the ornemental aspect of the cards will be mostly ignored, and the number of the suits as they are represented will be the only focus, as the pip cards ornementations and the way they articulate with the numbers deserve at least another whole text.
This should be studied in the same manner the I Ching hexagrams are studied, that is apart from their literary meanings - not that these meanings are not important, but the visual structure by itself can often bring enough information to interpret the results. In the I Ching hexagrams each of the 6 lines has a "function" not unlike the suits in tarot have a "general" domain of expression.
For instance in the tarot the Six is not expressed or represented in the same way in Coupes, Deniers and Batons. So one should tune his numerological views according to each expression of that same number.
It's quite different than standard playing cards where for instance 7 is represented in the same manner in spades, diamonds, clubs or hearts.
Spanish suited or more generally latin suited standard playing cards also offer this feature, but in a different way than tarot.
In the Tarot 7 is in the Deniers suit, represented - as in standard playing cards - in the shape of the geomantic figure Albus/Rubeus (as not everybody agree on the rightside up state of this card nor does everybody in the geomantic domain agree on the geomantic albus/rubeus pair orientation).
(note : this is not the case for every "Marseille" type tarot deck, for instance Ignaz Krebs deck offers another disposition reminding the geomantic figures Laetitia & Tristitia ; the Albus/Rubeus pair is nevertheless carasteristic of the majority of the Marseille pattern decks, from the oldest to the more recent Italian heirs)
Apart from the geomantic interpretation, there's a structural similarity with the four and three of deniers, 7 can be read as 3+4, which may be significant as quantities, or through its relations to the 3 and 4 in the whole reading.
(note : the seven of Deniers has not the same structure in all tarot packs, for instance the Ignaz Krebs tarot does not follow the more standard pattern that is found in the majority of Tarot de Marseille and in the Vieville pack.)
The same number (7) in Coupes is a lot different, it's a line of 3, a single cup, and a line of 3, so as numbers it can be read as 6+1 or 3+1+3
and by its graphical structure it is closer to the 8 of cups (3+2+3) than any other card.
From this point of view the 8 of Deniers and 6 of Coupes are quite structurally similar.
Swords and Batons emphasize the succession of odds and even numbers, bot not in the same fashion : in swords the odds offer a realistic representation of a sword (or a pair of swords for the 10) that reminds the Ace and differs a lot from the more abstract representations of swirling or curved swords, whereas in Batons, the odd baton is quite similar to the rest of the image. Another difference lies in the woven pattern which is external in the swords and central in the batons. Watching only the suits representations, we can see that swords adds two pairs while the batons tend to divide : in the example of the 7, in swords we have 3+1 central +3, in the batons, we have a centrifugal explosion of two pairs of three and a one less ordered, and we can read in the half batons how it may develop in 2+1+2+2+1+2.
So numbers are indeed full of information, but we have four materialization of each number from 2 to 10 in the numeral cards, it's not only about the numerological interpretation of each number in each suit considered as a domain.