In design view, click the WebChart flash element to select it.
In the property inspector, click the Edit In XML Editor button
In the XML Editor window, select the yaxis node then click the plus button and select "ticks"
this will add a ticks tag to the chart XML in the yaxis node:
edit the ticks tag to add a number property wit the total number of ticks you wish to show. for example to show 14 ticks: