Concrete always cracks. Well, that’s not quite true in practice, but it’s definitely true in principle. Concrete, when used as a building material, is intentionally designed to crack.
There are two reasons for that. First and foremost, concrete shrinks when it cures. If there’s something holding the concrete in place, like reinforcement, then it will crack around that. Sometimes visibly, sometimes with microscopic cracks.
The lines that always show up in slabs and sidewalks are designed to hide away that crack and prevent surface damage:
The second type of cracking happens when concrete is reinforced with steel. In reinforced concrete design, the steel is supposed to carry tension while the concrete carries compression.
The concrete that’s around the reinforcement isn’t designed to hold tension – it’s supposed to hold the reinforcement in place. When a concrete structure gets any significant load, it will have “tension cracking” where the concrete breaks and allows the reinforcement to start carrying load:
Now, there is some unwanted cracking that can occur as well. In particular, since concrete is always supposed to carry compression, cracking that goes all the way through a slab is a bad sign:
That sort of cracking suggests that different parts of a slab are moving in different directions, or that the slab got overloaded somehow.
There is also cracking that spiders out from a single point – those sorts of cracks suggest there’s a problem with the soil supporting the slab:
What this answer is trying to show, in a pretty roundabout way, is that there is no single answer to this question. Depending on the design, application, and installation practice, your concrete could be as thin or as thick as you want it and still wind up cracking.
There are some rules of thumb for about how thick you need to make things like garage floor slabs to prevent the bad kinds of cracking, but they vary a little bit from application to application.
And even then, improper construction can still lead to a bunch of cracks.