Basic Trope: The hero bursts in to fight the villain, and when they seem to be headed for a mano-a-mano Duel to the Death, the villain calls for his Evil Minions and a melee erupts...
Straight: The hero faces down the villain and the villain summons his Mooks.
Exaggerated: The villain summons a ludicrously vast army of minions, or else multiple large waves thereof.
Downplayed: The villain brings in a small squad of guards, and/or uses them sparingly in the battle.
The villain happens have guards around to summon because he is a warlord, mob boss, demonic overlord, etc. in the heart of his stronghold, and thus would have no reason not to call in backup in the event of a fight.
Or the villain is insufficiently Badass to stand up to the hero in a fight, and thus needs the minions' help to prevail.
Inverted: The hero, not the villain, is the one calling in the underlings.
Subverted: The villain summons his guards, but they are utterly useless in the fight.
Double Subverted: The villain summons his guards and they promptly prove to be useless, but then they come in critically handy at some crucial point later in the battle.
Parodied: After the hero takes down the villain's personal guard, he calls upon a second squad... And then another one... And another one!... And another one... And ANOTHER one... AND ANOTHER ONE! ...And so on, and so forth until he runs out of competent fighters, and starts sending out the janitors, the cooks, the maids, and all the other members of the maintenance staff to fight!
Zig Zagged: There are underlings around for either side; sometimes they get summoned, sometimes they don't.
Averted: The villain is facing down the hero but he is unable to call in his guards. Either because...
A) There might be a breakdown in communications, whereby the Mooks do not receive the order to join the fight and fail to realize that their overlord is in danger.
B) The guards may be blocked from entering the fight by some means.
C) Harshest of all, the hero may have already massacred the villain's guards outright.
Enforced: The villain originally wouldn't have bothered to summon his guards (or didn't have any), but changes were demanded and implemented which resulted in the guards being called in either to heighten the tension of the battle or to make the hero seem that much more awesome.
Lampshaded: Upon facing down the villain, the hero inquires as to whether the villain will be calling in his guards.
Invoked: The villain not only calls in his guards, he does so verbally and makes the swerve mid-sentence.
The hero expects that the villain will bring in hordes of minions when they finally meet to do battle, and so he specializes in weapons and/or training that give him the advantage when he is grossly outnumbered — or else the hero concocts a plan to trap the minions when they are summoned and either neutralize them or turn them to his side of the battle.
Double Exploited! The villain realizes that the hero will be ready for his horde of minions, and so he prepares himself by becoming an expert at one-on-one combat, and uses his Mooks merely to prevent the hero from escaping the fight.
Discussed: The hero and villain banter back and forth as to whether the villain will call in his guards, and the situations that may arise in either case. May involve head games by one or both parties.
Conversed: Multiple characters are watching a movie or show or playing a video game, and one of them calls when the villain is about to summon his Evil Minions — either through having already seen the sequence or just predicting it out of luck and genre-savviness.
Played For Laughs: The villain brings in his Mooks, but the fight turns into a debacle because of their comically inept attempts to bring down the hero.
Played For Drama: The hero has enough left in himself to take down the villain. With the additional guards, though, he's not so certain he can win.