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I originally created the markers with numbers in case there was a need to say "I want to cast a level 1 spell, but using a level 3 spell slot". I've been told that's silly, so the numbers aren't important.
Each spell level (1 - 9) has the appropriate number of spell slots, so you'll need a full set of markers for each spell level you need for your class. For example, level 1 spell casters need up to 4 markers because they can memorize up to 4 level 1 spells (depending on the class).
You can print all 22 markers to have one marker for every spell slot in the tracker, or you can print only the markers you need for your class - depending upon how many spells you can use/memorize for each level.
Hope that makes sense.