Looking at the Dungeon Worlds's Wizard playbook Prepare Spells move, it seems quite clear how it works at the first reading:
When you spend uninterrupted time (an hour or so) in quiet contemplation of your spellbook, you:
- Lose any spells you already prepared
- Prepare new spells chosen from your spellbook whose total levels don’t exceed your own level +1
- Prepare your cantrips which never count against your limit
So, assuming that wizard reaches 9th level (not very realistic in DW, but just for sake of argument), and decides to take one of the 9th lvl spells, he will have two spells prepared – his 9th level and some other 1st level. If he had chosen proper moves while advancing, he could possibly prepare 9th level and 3 1st level spells, but that's it.
Do I understand it correctly, that regarding spells, wizard can choose from very limited amount of spells prepared at once, unless he reaches high level and decides to stick with only with low-level spells?
Your numbers are right, but consider the spells.
Especially when you get up to 7th- and 9th-level spells, most of the spells are things you're going to be casting at your leisure, so that you can take an hour to prepare before or after using them.
Spells at those levels that you'll be casting in the middle of danger, like dominate and soul gem, are tremendous game-changers, and the price you have to pay for that power is giving up the broader utility you might want to have going into danger in the first place.