Maintaining a grapple is normally a standard action that affects a single target, as indicated by this sentence in the grapple rules:
If you do not release the grapple, you must continue to make a check each round, as a standard action, to maintain the hold.
Note how it says "the hold", singular. It does not say that doing this also maintains all other holds, so it doesn't.
Any grapples not maintained on any turn after the first automatically end, so normally you can only maintain a grapple against 1 target.
Greater Grapple and Rapid Grappler can increase the number of grapples you can maintain by expanding which action types can be used to maintain the grapple. Likewise, the Grabbing Master feat can increase the number of grapples you can maintain by letting you maintain 2 grapples per action spent (which also further cements the idea that you normally can only maintain 1 grapple per action). However, Grabbing Master doesn't let you start multiple grapples by itself, you need to already have something like Greater Grapple, the Grab special ability on a full-attack, or Snapping Turtle Clutch to be able to start a second grapple without losing the first.
Mimics do not lose grapples they fail to maintain (due to their Adhesive ability), but otherwise follow the same rules as above. That might indeed make it somewhat difficult/impossible for them to move after getting multiple creatures stuck, but that's why they can still release the adhesive themselves.