Do not create a table for each user!
Probably you need something like the following tables
Cards to Lists (if a card can be on multiple lists)
Hard to know without understanding your requirements.
You should not really run into performance issues but you'd start to look at partitioning. Again, without knowing, partition by user ...
Every piece of software/hardware has a limit. I think you can come up with a schema to store users, lists, and items, that is not the problem.
The second part is how to scale, which is the real problem. There are two ways to scale, horizontally and vertically. In this case, horizontal scaling is probably the best choice.
For example, one could split the ...