Why do some control circuits of star delta starter use two normally open auxiliary contacts of the main contactor and others use one? To me only one normally open should be enough. Can a contactor have more than one normally open auxiliary contact?
You can use as many aux contacts as you need. In many control systems, it is common to have at least two aux contacts, one for status back to the control system, and one for interlocking or safety.
Most of the starters I buy have two normally open aux decks and two normally closed aux decks on the contactor, and then another set on the overload relay.
If there is a circuit breaker feeding the starter, that typically also has an aux deck mounted to it. Better decks on breakers can distinguish between an off and tripped condition.
Extra aux decks just snap or screw on to the contactor frame.
Do you have a more specific question about starter wiring?