Hmm, this is a very interesting topic
It seems that I am one of very few who think quite the opposite regarding large spreads. I have since I began learning Tarot almost exlusively used larger spreads, such as the Celtic Cross, The 12 and 18-card versions of the Astrological spread, The Opening of the Key, Vala Cross (from the book Tarot Decoded which also contain the 18-card Astrological spread) as well as a few others, including Arthur Waite's 42-card alternative method of reading the Tarot, although I freely admit that this last one is hard to read.
I never feel any form of information overload or that the cards overwhelm me or such things. Quite the contrary, I feel that it is very enlightening to have more cards to help give me a good and thorough understanding of the situation I asked about.
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As to your theory/idea that large spreads "use up" the cards so that there will not be enough of them to accurately fill the remaining positions, my personal view, is that there are several ways to look at this.
You may possibly be using modern scientific reasoning (mainstream as it were), to evaluate or analyze a method that quite likely cannot be explained by modern scientific principles. Nothing wrong with that in and of itself, but I wonder if Tarot really works by these principles
We don't quite know how the Tarot work and indeed why it work. But just about everyone who put some genuine effort into learning the Tarot, will see, and as time goes on, learn and know for certain that it does work. What are the principles, and for a lack of a better term, supernatural laws, that makes the cards appear in the correct position as we deal them out according to our question and our selected spread? Not an easy question to answer. Apparently, however large a spread we use, the cards will make sense one way or another. Perhaps there is no law governing this at all, but rather a form of an, as of yet, unknown intelligence.
There is also another aspect to consider, namely that of intuition and psychic skills which play an important role when interpreting the cards. Quite often, ideas coming from intuitive/psychic insight, will tell the reader that a card may indicate something one would not normally consider correct as related to the card's divinatory meanings. Yet it may turn out that one's intuition was correct after all.
And of course there is also the interplay between the cards. A card may mean something different depending on its surrounding cards, and therefore the scientifically theoretical possibility that the Tarot may run out of available cards is, as seen in this respect, extremely small.
Personally I have never experienced a situation where the Tarot appeared to run out of cards to accurately explain a situation, when I used very large spreads. They have always made sense, either alone or in combination with other cards supported by intuitive insight. And as a final note, in my personal opinion, a Tarot spread should be seen as one (long or short) coherent message, not as many small isolated messages coming from each individual card. So even at the very theoretical chance that the Tarot should run out of available cards, if lets say, one or two cards in an 18-card spread does not quite provide a 100% correct meaning, the overall message should still be correct.
And there is even more. If you make use of elemental dignities, reversed cards, numerological aspects, astrological associations, locational dignities, card counting and more, I cannot really see any chance of the Tarot running out of cards to use
Anyway, that's my thoughts on this
Atlas