How thickly can I install self levelling concrete to fix a tilted garage floor?
Yes, you can only use such products to a certain depth. This is due to shrinkage and lack of aggregate, which means that thicker applications become rather weak.
More importantly, though, is that those products are not intended as a finished flooring surface. They're soft and chalky when cured, and you're supposed to put actual flooring over the top. It will not withstand the rigors of garage life.
Secondly, your garage slab may have been intentionally poured with a drainage slope. This means you can't use self-leveling products without causing a puddling problem. If you're in a freezing climate this is a serious concern.
You'll need to pour a new actual concrete slab--either as a replacement or over the existing in a substantial thickness.
However, if your only goal is a sealed door opening, I'd install a tapered shim onto your door and a new astragal (seal). Much cheaper and just as effective.