Calendula readily self-seeds so you will likely end up with more plants growing in spring/summer where you last planted them. If you would like to save the seed to give to others or plant elsewhere in your garden they are very easy to collect.
Simply wait until the flower head has dried off and get collect the crescent shaped seeds once they are turning brown but before they fall completely off the flower head. If you wish you could put a paper bag over them after the flower has finished blooming to make sure you don’t miss this perfect window. However you will likely have so many flowers you can collect from that missing a few seeds won’t matter. Make sure the seeds are fully dry before storing where they will last in cool and dry conditions away from direct sunlight for at least 4 years.