Well, for Accuracy, Crit, Skill Speed, Spell Speed, and Parry materia require 7 melds before you can move onto tier 2, 8 tier II before tier 3, 8 tier III before moving onto tier 4, and 8 tier IV before reaching the max cap.
However, for Determination materia it's different. Det requires 5 tier I, 5 tier II, 6 tier III, and 6 tier IV.
53 is the max stat allotment for the Curtana. Maxing one of the first materia I listed would cap at 31, while maxing Det would cap at 22. Or you can break it up however you want, the choice is yours.
Also keep in mind, the higher you go up in each tier the lower your chance for a successful meld becomes. To a point anyway, the first few materia in each tier should be a 100% meld but after that you will have a chance to fail and can lose the materia (but not the Alexandrite).