How many subsets of $\mathbb{N}$ have the same cardinality as $\mathbb{N}$? 
How many subsets of $\mathbb{N}$ have the same cardinality as $\mathbb{N}$?

I realize that any of the class of functions $f:x\to (n\cdot x)$ gives a bijection between $\mathbb{N}$ and the subset of $\mathbb{N}$ whose members equal multiples of n.  So, we have at least a countable infinity of sets which have the same cardinality of $\mathbb{N}$.  But, we could remove a single element from any countably infinity subset of the natural numbers and we still will end up with a countably infinite subset of $\mathbb{N}$.  So (the reasoning here doesn't seem quite right to me), do there exist uncountably many infinite subsets of $\mathbb{N}$ with the same cardinality of $\mathbb{N}$?  
Also, is the class of all bijections $f: \mathbb{N} \to \mathbb{N}$ and a given countably infinite subset uncountably infinite or countably infinite?

$\Bbb N$ has only countably many finite subsets, but it has uncountably many subsets altogether, so it must have uncountably many countably infinite subsets (i.e., subsets with the same cardinality as $\Bbb N$ itself).
If $A$ is any countably infinite set, there are uncountably many bijections between $\Bbb N$ and $A$.
