Is it possible to create a rule or use the REST API or something to Decrease a field called "Number of tickets" in an specific account??

I have created the field Number of tickets in Accounts, I set up the number of ticket that my customer pay to me and I would like to decrease this number.

In addition, I would like to create an alert or task to remember the customer to buy more tickets when "Number of tickets" are less than 5.

Thank you very much for you help,

Alberto