There is a cap on how much of each stat a single item can have, but you can keep adding materia despite that - once you unlock advanced melding (I think it's a level 30 crafting quest? but don't quote me on that) the game won't stop you from adding materia even if they won't do anything. Failure destroys the materia and carbonized matter but doesn't do anything to your equipment, so don't worry about that.
(If you're going for the 'I got this' achievement, you'll want to use it to burn up all that level 1 materia that you have. Also I think you may be able to use the 'cowl' equips for it, so you only have to rely on the RNG for one materia rather than 3.)
And iirc if you're adding materia to an item that doesn't already add that stat, you can actually go through quite a few before you cap it out.
(If you're going for the 'I got this' achievement, you'll want to use it to burn up all that level 1 materia that you have. Also I think you may be able to use the 'cowl' equips for it, so you only have to rely on the RNG for one materia rather than 3.)
And iirc if you're adding materia to an item that doesn't already add that stat, you can actually go through quite a few before you cap it out.
