1. First you want to detect the location where the invoice number is. There must be something that each invoice share that can help you detect it. If they all have different ways of displaying it, simply see if you can detect any of the possible ways it's displayed.
2. After detecting where the invoice number is then remove all noise and crop it down to just the invoice number.
3. Now you got the invoice number only and can use OCR on that to read the numbers.
It's a little more complex than that, but it should help you a tiny bit.
1. First you want to detect the location where the invoice number is. There must be something that each invoice share that can help you detect it. If they all have different ways of displaying it, simply see if you can detect any of the possible ways it's displayed.
2. After detecting where the invoice number is then remove all noise and crop it down to just the invoice number.
3. Now you got the invoice number only and can use OCR on that to read the numbers.
It's a little more complex than that, but it should help you a tiny bit.