max value represented by bigint
A bigint is always going to support
-2^63 (-9,223,372,036,854,775,808) to 2^63-1 (9,223,372,036,854,775,807)
SQL Server TSQL does not define this as a constant, but it is always going to be -2^63 to 2^63 - 1
See the answer provided in this similar question. There is no way, as far as I know, to programmatically find the answer you're looking for.
Based on the comments you posted on another answer, this would allow you to only have to change your values in one place, as opposed to multiple places.