The tickmarks on the X axis of a line chart in ClarisWorks represent the names of your series. If you have no names for your series it will default to labeling them numerically.
Those numbers are still the names of a series and those names are divided by tickmarks. Therefore ClarisWorks will not change the number of tickmarks that are displayed, even though you have filled in the Step Size box in the Axes dialog.
It looks deceiving because there are numbers and it would look as if you could treat them as numbers. In reality they are names and can't be manipulated.
If you want to achieve this look, you can use a column to put names for the X axis and simply leave some of them blank.
For example if you wanted every fifth one labeled you could enter this column:
1
(blank cell)
(blank cell)
(blank cell)
5
(blank cell)
(blank cell)
(blank cell)
(blank cell)
10
etc.