Digit
Noun
すうじ
数字がのびるからといって、下ひんなきじをけいさいするのはよした方がいい。
Regardless of the increase in numbers, you shouldn’t publish vulgar news stories.
この子は、一万五千六百七十二けたの数字をあん記している天才じです。
This child is a genius who memorized a 15,672-digit number.
数字の十一は、私がバレーボールをする時にいつも身に付ける背番号です。
Eleven is the number I always wear when I play volleyball.
A number letter isn't a letter like the ones you're reading now, it's more like a numeral, like 1, 2, 3, 4, 5, etc.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.