I realise that a form as a whole can be disabled when an overall number of submissions is reached, but how would I go about implementing this feature for one field in particular.
For example, a series of classes may have a single entry form like so:
Dance (Max 30 participants)
Singing (Max 40 participant)
Instrumental (Max 50 participants)
... and so on.
When the max number of people have selected 'Dance', i.e. 30, then the actual option of entering in this is disabled.