Just looking at Followiz and JustAnotherPanel (some cheaper but fine panels) and they have minimums of 200 and 50 respectively, which means you could order in increments of 50. But what is even easier is that they have a feature called Drip-Feed, which as it sounds it just adds the amounts of followers every so often based on the interval you set. I don’t know the interval maximum, but you could probably leave some hours (not sure about days) between each one being added. Just bare in mind the longer intervals you add the more chance you add for the potential of the service to screw up half way through, which happens with some orders not doing full amounts. So you could always do 1000 over a few days, then another 1000 over another few days, for example.
Edit: With all that said, you would probably safely get away with just throwing on 2000 in one hit. But may as well use the drip-feed since it’s available, for safety.