For MySQL database:
In BIGINT(8), the number 8 represents how the data will be displayed. It does not affect the storage. The number is used to display width.
BIGINT takes 8 bytes i.e. 64 bits. The signed range is -9223372036854775808 to 9223372036854775807 and unsigned range takes positive value. The range of unsigned is 0 to 18446744073709551615.