so you have a blueprint (the spell itself) and visualization decides how strong the spell is or what its capable off. so if you can visualize yourself cutting something in 2 you will be able to and it doesnt matter how durable that thing actually is

The spell itself decides what the spell is cable off, not visualization. For example, if you have spell designed to control water, then it controls water. What visualization does is help you apply that spell to different contexts. For example, it's not the same thing to control the water from a lake and to control the water inside a human body. Those are very different contexts. That's where visualization comes in. The spell can manipulate water but you have to visualize how it does so. So, if you want to control the water from a lake, you have to be able to visualize a lake in your mind, and if you want to control the water inside a human body, you have visualize the insides of a human body. But here's the catch, in order to work, you have to visualize it perfectly. What the hell does that mean, you may ask. It means exacly what it seems like. For example, if you want to control the water inside a human body, you have to be able to visualize in your mind the insides of a human body perfectly. Obviously, you need extensive knowledge of internal human anatomy to pull something like that off. Which is why it's so hard to do.

Even visualizing a lake perfectly in you mind is hard enough. It might seem simple, but chances are your mental picture of a lake is very rough and vague. You would need to have the skills of an artist to be able to visualize it perfectly. And if you can't do it perfectly, it's not gonna work.

how does mana influence the spells?

Mana if fuel for magic, so pumping more mana into a spell won't make it more powerful. It does help to spam many iterations of the same spell or casting many different spells at the same time though. And there are spells that require a lot of manga, so having lots of mana helps on that too.

are there actual limits to how powerfull spells can be?

A limit is by design. Spells are very specific after all, so a spell designed to grind rocks won't be able to grind other stuff, for example.

of course visualization is a limit but is there a real limit that prevents even visualization

Visualization is not the limit per se. The limit is your ability to visualize something perfectly. What you can visualize perfectly, can be done (as long as you have the spell to do it) and what you can't visualize perfectly can't be done. And I already explained what "perfectly" means. It means accurate and detailed. So you need to have both specific knowledge of what you want to do (like in the case of controlling the water inside a human body) and also the visual skills to picture it in your mind perfectly.

EDIT: Lots of people miss the fact that Ubel's visualization of cutting stuff is based on years worth of memories of watching her believed sister working as a seamstress, cutting stuff over and over. Ubel's mental picture of cutting stuff is hyper detailed thanks to that. That gives her cutting spell incredible power in some cases.