hidden fields

hi
this is might simple to implement, but i cannot do it.

here is the scenario. i have a form that a client uses to submit information about activities. the maximum number of activities are 6 e the minimum 1.

so the client has to be able to choose the number of activities, say 3 and the form with fields for 3 activities will load for him to fill in. i do not want to show all 6 possible activities if the client needs to fill in the fields for 3 only.

how can i do that?

thanks