How does a Pareto efficient equilibrium scale for n individuals? Say I have a starting situation with Pareto efficient allocations between 2 individuals, now a third one comes in and changes the allocations. Now a fourth one, etc. Up untill a nth individual comes in. Do we now have to resolve (n-1) equations or n^(n-1) equilibria? How does the "difficulty" scale for n individuals? I don't know if that makes sense? Thanks.
If you are doing pairwise comparisons, then with $n$ individuals there will be $$\dfrac{n(n-1)}{2} $$ such comparisons.  You can write this as $\binom{n}{2}$ or as $\frac12n^2-\frac12n$ if you prefer.  

$1$ for two people, $3$ for three, $6$ for four and so on, known as triangular numbers