Pillagers keeps spawning if player is nearby within sec 7 to 9 are spawned in a little bit more than a half minute if u go kill them all with command it will still spawn i have tested this is client and server side

how to check this urself

1. create a world click none just make it (or test on ur server)
2. go to outpost
3. see how they spawn from a other players view whille u are killing them or with command in survival and creative this wont matter they still spawn this only not count if in spectator mode 

this video was after the high spawn rate it goes randomly but here will be atleast shown the speed of spawning again this still can be randomly