how do i find the id number of a film

Well, there's something called an ISAN, which you can read about here, but I don't think every movie necessarily has one:

https://en.wikipedia.org/wiki/International_Standard_Audiovisual_Number

Was there a particular question you had about the 1939 "Hunchback of Notre Dame" that someone on the message board might be able to answer?

Now the IMDb does assign every movie its own number, which is imbedded in the URL for that movie, but I doubt it has much application  beyond the IMDb. Here's the URL for "Hunchback":

https://www.imdb.com/title/tt0031455/

So the IMDb number would be tt0031455.

ISBN numbers are assigned to each edition of a published book.

The closest thing would be the UPC assigned to a physical product like a dvd or blu-ray disc (or a can of soup; books also have a UPC barcode to facilitate purchase).
For Hunchback ('39), bn.com lists
0883929151387 for the 2010 dvd
0053939205824 for the 1998 dvd
0883929447527 for the 2015 blu
0053939637731 for the 1996 vhs
There may have been other releases, which would have their own UPCs.

Amazon does not display the UPC on their product listing - they have their own version of a product code they call the ASIN. I don't think any non-Amazon sites would use an ASIN. However, you can search for a product by UPC on Amazon, so they do store that data, just don't display it (unless they show an image of the product that includes the UPC). 

In Europe, the equivalent idea is EAN. 
International Article Number - Wikipedia

IMDb does not appear to have UPCs in the database. At least I've never gotten a result by searching the UPC within IMDb. But I gave up pretty quickly after trying only a few. I've never run into a place to enter a UPC or that displayed a UPC, unless someone included it in a trivia item.