One of the tropes of martial arts movies is that there is essentially no such thing as “strength in numbers”; it’s an important aspect in which movies which follow this sort of concept depart from any sort of realistic combat situation, in which being outnumbered in a fight is, all else being equal, something of a disadvantage.
It comes down to the purpose of the fight in question. If the point is to establish a character as a badass without depicting them defeating an actually important character (because it isn’t the appropriate time in the story for them to do that), then having the character mow down a swathe of nameless mooks serves that purpose. If you want a showdown between a protagonist and an antagonist, you want it to be a one-on-one fight so that the spotlight is entirely on the main characters. Sometimes a movie is artful enough to set this up plausibly, sometimes it requires a bunch of trained fighters (who as part of that training presumably have been encouraged to overcome the instinct to flinch back and not take part) to stand around doing nothing whilst the hero kicks their boss’s head off.
The cheaper sort of ninja B-movie that was all the rage in the 1980s is an example of precisely this sort of thing – especially the sort of material made on the cheap by Western filmmakers who didn’t really understand the genre but wanted to ride the bandwagon. One studio which had no qualms about doing that sort of thing was the Cannon Group, particularly under the Golan-Globus era – in fact, their loosely-connected Ninja Trilogy (which, despite the name, essentially consists of three entirely disconnected movies) is often credited with starting the 1980s fad for American ninja movies, though it was predated by the Chuck Norris vehicle The Octagon.
Now, we know that the more ninjas participate in a fight, the less effective they are. Is that also true of film quality? Do movie franchises get worse the more ninjas you stuff into them? Let’s find out…