What are you playing on? On PS4, just swipe on the touchpad to expand the map temporarily.
Yes, the goal is just to get to the bottom and beat the end boss. You unlock more and more things as you go, including more things to do in the game, more characters, more items, more bosses. As for how many levels...let's just say you can't get to all of them yet, but focus on beating the levels the game offers you.
Items are complicated. There are a LOT of them. Most have fairly obvious effects like stat ups, some...really don't and you might just want to use
http://platinumgod.co.uk/rebirth to look up some of those. I'd personally recommend just playing and learning on your own, only looking up items you completely don't understand, and if some specific mechanic is completely puzzling you, ask in here or look it up (there is a complete wiki for the
original game and in-progress wikis for the current one).