URGENT! Display Calculation using Drop-down & Number

Hello!
Sorry for posting this again, but it’s quite urgent!

I have a user input form which is connected to a database.
The form includes a.o. the following fields:

  • A drop-down button to ‘select a date’ (which has 6 options)
  • A field to input a ‘number of tickets’ (minimum = 1)

Every time the form is submitted, I want the displayed ‘number of tickets per date left’ to change accordingly. The total number of available tickets per date is 86.

Unfortunately, I don’t have enough experience to code this.
It would be great if someone could help.
Thanks a lot!