You probably have the Customer as entity $Customer available in your microflow and your Movie as $Movie.
It can also be that you are looping over multiple Customers and then you use $iteratorCustomer.
Create Variabele with type decimal and name this CalculatedTotalPrice.
Within this action, you add “$Customer/Number_Of_Tickets * $Movie/Price”
Then change the Customer entity with Change Object action.
Set parameter Total_Price to $CalculatedTotalPrice
On the page, if you want to calculate it when user fills in the Number_Of_Tickets, use the onchange event on the textbox.
Connect that to Microflow action in which you do the calculations as described above.
Once user enter number of tickets call a microflow on change event , in this microflow retrieve movie and then create a variable of decimal type and in this variable multiply the number of tickets and price and store the result in the variable. Now use or store this created variable value where you want.