Are certain tasks given to only one crewmate maximum?

No, I’ve seen multiple people have those tasks.
Also if there were a lot of tasks like that, just think how many different tasks there would have to be of similar length.

There are no tasks in Among Us that are only given to one person. There is always a chance that only one person gets a certain task but there is never a 100% guarantee.

There are however, tasks that all crew mates get. These are called common tasks and everyone gets them. For example, wires is a common task. So if you have wires then everyone else in the game has wires, even the imposters will have wires on their fake task list. But if you don't have wires that means that no one else in the game will have wires.

No - The factor that actually determines whether someone will have the same task is the settings related to number of tasks, and in particular, number of long tasks.

1 long task is a fairly common map setting. The Skeld for instance has 7 long tasks: Clear Asteroids, Empty Chute, Empty Garbage, Fuel Engines, Inspect Sample, Start Reactor, Submit Scan. With 8 crewmates, if you only have 1 long task to do, there is a good chance (but not a guarantee) that you will be the only person with that particular long task. Though there is a guarantee that at least one of those tasks will be assigned to two or more crewmates.