There are two types of comic books: mini-series and ongoing series. This is actually relatable to television mini-series (like "V") and long-running series (like "Star Trek: TNG"). So Thor: Deviants Saga is a mini-series, meaning there are only 5 parts, and the series runs alongside and independent of the Thor ongoing series, called Mighty Thor which is currently on #11 (and will be continued to be published until sales decline or etc). Green Arrow #7, which does not state something like "7 of 12," indicates it is an ongoing series and has no set end point.
The best way to find out if a comic book is starting a newstory arc or is a good jumping on point is to read the summaries or investigate the covers. In the case of Green Arrow #7, I'd say that is a good jumping on point because the summary says
Join new creative team Anne Nocenti and Harvey Tolibao as they chart
the next chapter in the life of Oliver Queen!
Summaries will also sometimes indicate "Part 1 of xxx Storyarc!" Covers will sometimes say this too, or they will say something like "First issue in a bold new era!" or "New beginning starts here!" to note that even though it is not a #1 issue, it is a jumping on point. The comic book companies know they have a daunting back catalogue that can be off-putting to new readers, so they really try to advertise the jumping on points. Another thing that Marvel specifically does, they publish Point One issues, which are intended to be jumping on points. So for instance, if you see Amazing Spider-Man #679.1 or Thunderbolts #163.1 then that means that, hopefully, it's a standalone issue that is meant to be a jumping on point. This isn't always the case with Point One issues, but it is the intention.
So to sum up, "#3 of 5" means a series is a mini, and telling one complete story in five issues. #3 means it's the third issue of a series that will keep going until it gets canceled.
The best ways to tell if a comic is a good jumping on point are:
- If it's a #1 issue, either mini-series (#1 of 5) or ongoing (#1)
- If the summary declares it has a new creative team or starts a new storyarc
- If the cover itself boldly states it's a new direction, new beginning, new storyarc, etc
- If it is a Marvel Point One issue