How many electrons can have the quantum number set n=5, l=3, ml=-1?
This post answers the question of the of what the other quantum numbers mean:

What do the quantum numbers actually signify? 

As for how many electrons can have $n=5,\ l=3,\ m_l=-1$...

The Pauli Exclusion Principle states that two identical fermions (an eletron is a type of fermion) cannot occupy the same quantum state. 


  In other words, two electrons (in the same atom) cannot have identical quantum numbers.



For each $n$, the values of $l=0,1,2,...,n$
For each $l$, the values of $m_l=-l,...,-1,0,+1,...,+l$


For each $m_l$ there are only two allowed values of $m_s$, namely $m_s = + \frac{1}{2}$ and $m_s = -\frac{1}{2}$. Thus, each set of $\{n,l,m_l\}$ can only describe two electrons. In your case, we have two electrons with $n=5,\ l=3,\ m_l=-1$:


$\{n,l,m_l,m_s\}=\{5,3,-1,+\frac{1}{2}\}$
$\{n,l,m_l,m_s\}=\{5,3,-1,-\frac{1}{2}\}$