There are actually four words for "number": nummer, siffra, antal and tal.
There is another word for "siffra" in English and it is "digit". There are ten "siffror": 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. You can combine them in various ways to make "tal".
"Nummer" is a number that identifies something, like a social security number, an account number, a telephone number et c.
"Antal" = quantity