How do I know the maximum value of
You can use an aggregate function in order to get the maximum value in a column
SELECT MAX(myColumn) AS MyColumnCount FROM myTable
If you want to know the maximum value for a customer for instance, you can group by this customer
SELECT CustomerID, MAX(myColumn) AS MyColumnCount FROM myTable GROUP BY CustomerID
You can find other aggregate functions here.
If, by contrast, you are interested in range of a decimal type, then consider, that you are declaring the total number of digits and decimals. Therefore the maximum number is reached, when all these digits are
decimal(5,2) it would be
5 is the total number of decimals to the left and to the right of the decimal point.
2 is the number of decimals to the right of the decimal point.
The maximum possible range for decimals is -10^38 + 1 through 10^38 - 1.