Finding Max Value in a Row

I have a problem in SQL I don't know how to solve and while I'm sure
there are 100+ ways to do this in ANSI SQL, I'm trying to find the
most cleanest / efficient way. I have a table called 'users' and the
field 'users_id' is listed as the PRIMARY KEY. I know I can use the
COUNT function, then I know exactly how many records are listed but I
don't know what the maximum or highest numeric value is so that I can
use the next available # for a newly inserted record. Sadly the
architect of this table didn't feel the need to create a sequence and
I don't know how to find the highest value.

Thank you for any assistance!