I started off small - first a single AP at my desk for testing, then half a dozen APs in our office area. Then a small building (~6 APs), worked my way up. I had no qualms moving large buildings with 100 APs to 9880s all in one go. These controllers can reboot and have hundreds/thousands of APs join them and start downloading code (if it wasn't predownloaded ahead of time). So, from the 9840's perspective, moving a bunch of APs at once to them shouldn't be a problem. It's more your own decision about risk management. The fewer APs that move at a time, the less angry people you'll have if something is missing or doesn't work on the new controller. We have very few buildings/APs with oddball configs, so the more that move over, the less risky the moves become, assuming you have your site tags configured per best practice (see below).
Cisco Prime made choosing which APs to move and to which controller and when (scheduled for a later time) made the migrations a breeze.
Otherwise, standard practice of keeping adjacent APs on the same controllers applies. Do entire buildings at a time, or at least entire floors or wings of buildings, if possible, to minimize roaming between controllers (even if they're in a mobility group together); avoid "salt and pepper" situations.
Most importantly, if you haven't seen this, spend some time with the 9800 best practices guide and make sure your site tags, etc. are to spec: https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/technical-reference/c9800-best-practices.html
Once you have your configuration completed and tested, you can run the config through the config analyzer and it will suggest further improvements as needed: https://cway.cisco.com/wireless-config-analyzer/
