Is there a word for variety of letters used in a language?

Question

To suggest either/both readability or efficiency in typing?

English is less [is there a term to fill in here?] than Japanese: compared to English alphabets, the number of letters in hiragana + katakana + kanji set are much larger and each of them is more distinctive (the bigger size of each letter allows more strokes to make it more distinctive from other letters). This makes words shorter and of more readability while harder to type since you have to convert alphabets into kanji etc.

English is more [the same term used in the blank above] than some hypothetical language where you spell every word using only "p"s and "q"s (thus the words tend to be longer with less readability; probably easier to type since you have to use only two keys).

Answer 1

The terms encoding density, information, and entropy circumscribe the property you are aiming at.

When you replace your p's and q's with zeroes and ones you are at the bits computer science deals with. Computing the entropy of the English writing system is a typical exercise in theoretical computer science (Note that not only the number of character but also their frequency counts).