That is a very complex series of questions, so buckle up.
Most publishers will publish at most 1-2 major hentai magazines per month. Here are a few examples, but keep in mind that even these publishers are on the bigger end:
Core Magazine is most well-known for publishing Hotmilk and Megastore Alpha, but they also have several smaller magazines.
Angel Publishing just has ANGEL Club, but it's rather long, about 460 pages, and they've been publishing that one magazine since 1999.
Akane Shinsha is known for multiple magazines, including Tenma, KOH, LO, and Girls forM. However, there are only two issues of KOH and four issues of Girls ForM released each year.
Wani is actually the outlier in the grand scheme of things, having four major monthly magazines (X-Eros, Shitsurakuten, Kairakuten, Kairakuten BEAST) in addition to lesser magazines (Kairakuten XTC, Hana-man). X-Eros usually clocks in at over 500 pages, making it the longest hentai magazine by a fair margin.
In terms of content, I'd estimate maybe 40% of magazines are more hardcore than X-Eros (Megastore Alpha, ANGEL Club, Shingeki, etc), 20% about as vanilla as Kairakuten (KOH, Monthly Vitaman, Action Pizazz, etc), and 30% are in between or have too much variety to say for sure. For "themed" magazines, there's Unreal and anthurium, which are basically that 40% but with more fantasy/sci-fi settings, and Girls forM, which is basically that 40% except without sex and more balls being literally kicked. LO and Otokonoko Heaven are basically vanilla loli and vanilla traps, respectfully.
As for the best magazine, it's Otokonoko Heaven. Make sure you let Jacob know.