Hi Bianca

Custom signup fields are applied site wide rather than per seminar so aren't able to be limited to selected seminars.

One workaround is to create a text field - make this not required and the default value blank

Add Regular expression validation for example limit input to 9 numbers by using /^[0-9]{9}$/

Add some text in 'Description of regular expression validation format' stating that the contact number is required if you are signing up for the following seminars 

The user will have the phone number field show when signing up with instructions to fill this in if they are signing up for the following seminars.

Regards