The spawn chance ranges from 2.0% to 3.5% in increments of 0.5% depending on the number of people in the squad who have any mark from Stalker (does not need to be the same boss). Downing him does remove marks, but you can only have one mark at a time from any given boss (note that Phorid, Lephantis, and the Zeloid Prelate all count for marks just like other bosses, but Corrupted Vor does not, not sure about the Jordas Golem). The exact number of marks you have has no impact on his spawn rate, just how many times he’ll spawn before leaving you alone again.