Yes, 076 is a seat number. I would look around for a carriage number on your ticket too. There will not be a ticket on the seat.
403 is the sleeper carriage number (actually it’s a “couchette”). It’s labelled on the outside, and there will be several staff around to help when you board. Your ticket should also indicate your berth number(s) (Liegeplätze), probably formatted with a leading zero and perhaps under “Oben”, “Mitte” or “Unten” indicating whether it is the top, middle, or bottom bunk. For example, in my compartment of carriage 403 last week the berths were 041 – 046.
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