No manga artist does anything by their selves now a days. It's all done as a team, and a big one at that. Somewhere between 5-15 people for the average manga artist and 20-30 for the big three.
It's just not possible to create a good manga(anymore) by yourself. Even Akira Toriyama(The god of drawing) struggled to put out Dr. Slump volumes, and that was back in the 80's. And even the
master still had a small team.
I am wondering about this system... shouldn't these people get credited for their work? on the other hand the same system applies in other countries and not just for manga, but any professional who has a budget and deadlines to pressure him has a team of people doing some of the work.i do know that those who later on become mangaka themselves will usually mention it if they have worked for a big name, and i do believe that they will have other benefits this way; like learning some techniques, meeting with important people etc. but still...
Most popular manga magazines publish weekly, it's not uncommon in Japan. And I get my numbers from being Japanese and living in Japan, the numbers you read from your volumes probably include substitute assistants, editors and those who are given credit where credit is due. In reality authors tend to only have 2-5 permanent assistants that mostly do the work, which is still quite a lot compared to a decade or so ago as you said. Authors still do most of the work, they're often in charge of the sketch, name and the story, assistants are mainly there to refine the work by including backgrounds, tracing and adding other toucah ups, from what you said you make it seem as if authors do jack all.