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.
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.
数字をかく
to write a number
数字を入力する
to input a number
数字を学ぶ
to learn the numbers
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.