Numeral
Digit
noun
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.
すうじ
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.
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.