in IDA you search for sv_maxRate. When the search is done you will find something like this:
The 40h says that the max value of sv_maxclients is 64.
Now switch to hex view and click on the 40. Now you will see the offset at the left bottom in IDA:
In your hex editor go to the offset and change the 40 into 80.
(the offset in the screenshots is for 1.0)