1 How to login federated arcgis server manager through arcgis portal admin
portal admin:
https://gisportal.test.hrsa.gov/server/admin
2 change max return count parameter as what ever you like depends on how fast your physical server CPU and RAM memory your machine had versus how much data your layer have and SQL server performance etc..... It is a complex number, all kinds of factor play in. The best practice is you try 50, 100, 200, 300, 500, and so on, until you see your server significantly slowing down. Then you select the number right before "slowing down". Or make it simple, just use 100, or 200, idiot proof.
ESRI document:
portal admin
https://enterprise.arcgis.com/en/portal/latest/administer/linux/administer-a-federated-server.htm