Invoice numbers in planmanager are linked to the journal. You can have multiple journals.
You can find a list of all your journals in the Invoice menu - Journals.
Click on a line to edit a journal. The last number field holds the last invoice number that was distributed. This means that if the last number is 1003, the next invoice will be numbered 1004.
On January 1, I change my numbers for the New Year
Your current number sequence is 201700605. However, you want the first invoice of 2017 to start at 201700001. You can simply modify your existing journal by putting in 201800000 to number your next invoice to 201800001
On January 1st I change my numbers back to 1.
An invoice number must be unique by definition, did you use for example invoice numbers 1 to 605 in 2017, you can’t use these numbers in 2018.
You will then need to rename your journal in 2017 and create a new journal by clicking the new button. That way you can use the same invoice numbers again.