Did you look at the example in the jqGrid Demos page? The answer is there.
Open http://trirand.com/blog/jqgrid/jqgrid.html
Open Chrome Developer Tools
See what is the URL when you click the next page button on the grid?
If you click the previous page:
As you can see the page value in the query string change from 2 to 1.
Then, see the PHP code. The page variable is use to retrieve data.
That means if you give a number of rows, the grid does not calculate page number automatically. You should do the calculation from the CFC.