Oracle Table Design, Storing Small Values
The documentation states the size of datatypes here.

0 (the number) is stored as 1 byte. Other single digit numbers will be stored in 2 bytes (one for the exponent, one for the mantissa). You can test this yourself by creating a test table and using the VSIZE() function on test data (doc link).

A CHAR(n) will be stored in n bytes.

I'd always store numbers in a NUMBER. Unless you're dealing with trillions of rows, the space saving will not be significant enough.