Why do you think a grimoire consists of pages of paper?

Because every page only contains ONE RUNE!

The shape of the rune must be drawn in precise manner, lest the spell power is diminished, or worse, backlash to the caster in a spectacular manner.

Beside that, each page is thick to allow the magic ink to properly seep to the pages and withstand the aging.

If a spell requires 20 runes to be properly cast, then you can see why a grimoire can only contain one spell. Or maybe, one half of a Summon Meteor spell?

Why do you think a grimoire consists of pages of paper?

Because every page only contains ONE RUNE!

The shape of the rune must be drawn in a precise manner, lest the spell's power is diminished, or worse, backlashes to the caster in a spectacular manner.

Besides that, each page is thick to allow the magic ink to properly seep into the pages and withstand the aging.

If a spell requires 20 runes to be properly cast, then you can see why a grimoire can only contain one spell. Or maybe, one half of a Summon Meteor spell?