Numeral, Digit, Number
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.
数字を入力するのが下手です。
I'm not good at entering numbers.
その数字を見て、答えを思い出しました。
Looking at the numbers, I just remembered the answer.
アラビア数字の８の形が、一番すきです。
The shape of the Arabic numeral 8 is my favorite.