Are every 2 DFAs with $n$ states for a language $L$ isomorphic to each other?
Let $M$ be a DFA for the language $L$ and let $M_1$ and $M_2$ be $M$ with two different collections of some number $m$ of unreachable states added. $M_1$ and $M_2$ both have the same number of states and both accept $L$, but they need not be isomorphic.

For a less trivial example, consider the following two DFAs:

Start state: 1
Accepting states: 2, 4
1 -a-&gt; 2   1 -b-&gt; 1
2 -a-&gt; 3   2 -b-&gt; 2
3 -a-&gt; 4   3 -b-&gt; 3
4 -a-&gt; 1   4 -b-&gt; 4

Start state: 1
Accepting states: 2, 4
1 -a-&gt; 2   1 -b-&gt; 3
2 -a-&gt; 3   2 -b-&gt; 4
3 -a-&gt; 4   3 -b-&gt; 1
4 -a-&gt; 1   4 -b-&gt; 2


These two 4-state DFAs are non-isomorphic, but both accept the same language, namely the language of all strings over the alphabet $\{a, b\}$ which have an odd number of $a$s.

Note that if two DFAs accepting the same language have the same number of nodes, and that number is minimal among DFAs accepting that language, then the two DFAs must be isomorphic. See DFA minimization.