If you click the “Edit G-Code” button on Mach3, you will see somewhere close to the top lines the words: S2000 M03. The S2000 means 2000 RPM for the spindle and the M03 means to start the spindle clockwise.You can change here the 2000 to whatever your spindle is capable or you can adjust the spindle RPM at the Config Menu / Spindle Pulleys as it was mentioned.
But be careful, the spindle RPM is also related to the Feed per minute of your cut among other factors. In other words depending on the material you are cutting there is a certain range of RPM / Feed rate. Google RPM Feed Rate Settings to see some tables with various materials / settings