Working memory span is mostly independent of item complexity

There are more than twice as many alphabetical letters as numeric digits, so you might expect that we could remember strings of digits about twice as long as strings of letters. But in fact, the typical Span of working memory is about {six} letters and about {seven} digits (e.g. Crannell and Parrish, 1957).

See: “Chunks” in human cognition

References

Crannell, C. W., & Parrish, J. M. (1957). A Comparison of Immediate Memory Span for Digits, Letters, and Words. The Journal of Psychology, 44(2), 319–327. https://doi.org/10.1080/00223980.1957.9713089

Miller, G. A. (1956). The magical number seven, plus or minus two: some limits on our capacity for processing information. Psychological Review, 63(2), 81–97. https://doi.org/10.1037/h0043158 Miller - The magical number seven, plus or minus two