Setting limits on the fields within your forms is simple with gravity forms.
Limits can be used to set maximum amounts on most fields. For example you can limit the volunteers attening the swim meet or in this example we are loimiting the number of children allowed at each tennis lesson.
To apply a limit go to the field that needs the limit. Select settings.
The settings will appear to the right of the screen under the "general" drop down.
Scroll down to the field type, Click enable Limits and set you limit.
Be sure to click Save Form in the upper right.