Welcome to FT, hijabi! You question is not really *A specific but anyway, I will give it a go.

If something is on one ticket, then this means that all flights on the itinerary will have one ticket number. In the age of paper tickets, this meant that all flights were in the same paper booklet and had one ticket number. Nowadays with E-Tickets it just means that all flights are under one ticket number.

One itinerary can mean one record locator of one of the big booking systems like Amadeus which can include multiple different tickets. So many different flights with some of them on different ticket numbers get one umbrella PNR (the record locator) which makes the entire booking.

If you purchased a round trip online and it is with the same airline then this will most probably be on the same ticket number. But as we say here: YMMV.