Each gang was only allowed to bring nine members to the big meeting, but as we see, even fourth-rate gangs like the Orphans actually have many more members than that. So why is it that when the Warriors finally make it back to their home turf they are unable to call on any extra help?
Because they're a small gang and don't have extra help. Their theme song is "Last of a Ancient Breed" and it's mentioned that no one thought The Warriors would ever make it home and no one in the Riffs (The larges and most powerful gang in New York) even knows who they are.. They only had nine members to send to begin with.
The game kinda jossed that. Mostly it's just more likely that they didn't have time to head back to their hideout for backup with the Rouges chasing em down as soon as they made it to the Island. It was dawn after all, and they were only awake because they'd been running all night.
The movie's deleted beginning scene does mention there are over 120 members in the gang, but it is entirely plausible that they had no chance to contact anyone for help and that, at that time in the morning, no other Warriors might be around the area.
Plus, on top of all that (odd hours, always on the run, etc), this is the era before cell phones. Meaning calling help requires a landline - rare unless you count a working phone booth and having change. And if their hideout is an old abandoned amusement park, there may not be a phone on the other end. Heck, they may not even remember the number.
They didn't have enough time to call for reinforcements, if any exist in the first place. From a storyteller's perspective, there are lots of reasons why it would be unsatisfying for this to happen. There'd suddenly be a bunch of strangers on the hero's side that we'd have no emotional attachment to. If the Warriors were to suddenly outnumber the Rogues, it would undercut the tension of their final confrontation. When the Warriors get complimented for their toughness and walk into the sunset, there would be a bunch of extra Warriors with them who hadn't experienced the events of the movie.