The only format here that plays by different rules
Every other format on this site runs in multiples of four. This one runs in multiples of two, because a broadsheet sheet is folded once rather than twice, so a single sheet yields two pages rather than four. That is genuinely useful. A six page paper is impossible on any tabloid format and perfectly ordinary here.
It is also the only format whose margin applies to the inside edge as well as the top, bottom and outside. So the printable width loses two full margins, thirty millimetres, rather than the one that the tabloid formats lose. At 380 by 578mm trim, the printable area comes out at 350 by 548mm.
Twenty pages is a hard ceiling
This is the constraint that decides whether the format is viable for your project, and it is much lower than the sixty-four available on tabloid. A broadsheet page holds a great deal of content, so twenty pages is more words than it sounds, but if you are planning a substantial publication you should check the arithmetic before committing to the shape. The words to pages estimator will tell you quickly whether your draft fits.
Designing at this size is genuinely different
A broadsheet is enormous, and layouts that work on a tabloid fall apart when they are scaled up. Column count is the first thing to reconsider: five columns across 350mm gives lines around seventy millimetres wide, which is too long to read comfortably in newspaper body sizes. Six or seven is more typical, and real broadsheets often run more.
The second is that a broadsheet is normally read folded, so the top half of the front page has to work on its own. That is why traditional newspapers put the masthead, the lead story and one strong photograph above the fold. It is a design constraint that simply does not exist on smaller formats, and ignoring it produces a front page that looks unbalanced on the news stand even though it looks fine flat on a screen.