Number of conjugacy classes in special linear group of fixed degree over a finite field is PORC function of field size
From Groupprops
Statement
Suppose is a natural number. Then, there exists a PORC function of degree such that, for any prime power , the number of conjugacy classes in the special linear group (i.e., the special linear group of degree over the finite field of size ) is .
A PORC function is a polynomial on residue classes  it looks like different polynomial functions on different congruence classes modulo a particular number. In this case, we only need to consider congruence classes modulo to define the PORC function. In fact, for a field size of , the polynomial depends only on the value .
General observations
Below, we list some general observations about the PORC function in giving number of conjugacy classes in .
Item  Value 

Degree of polynomial  for all the polynomials 
Leading coefficient of polynomial  1 for all the polynomials, i.e., each of the polynomials is monic 
Factors of polynomial  There are no common factors to all the polynomials 
Coefficients of polynomial  Is it also true that all coefficients of the polynomial are integers? (Seems so) Are they always positive? (seems so) 
Comparison between polynomials  Suppose are prime powers such that divides . Then, the polynomial that works on the residue class of is bigger than the polynomial that works on the residue class of , where bigger means that the difference has positive leading coefficient. In particular, the smallest polynomial corresponds to congruence classes that are 1 more than classes relatively prime to , and the largest polynomial corresponds to the congruence class 1 mod . 
Particular cases
(degree of special linear group, also modulus to which we need to consider congruence classes)  (degree of PORC function)  Possibilities for  Corresponding congruence classes mod for  Corresponding polynomials in PORC function of giving number of conjugacy classes in  More information 

1  0  1  1  1  the group is a trivial group 
2  1  1 2 
0 1 

element structure of special linear group of degree two over a finite field 
3  2  1 3 
0 or 2 1 

element structure of special linear group of degree three over a finite field 
4  3  1 2 4 
0 or 2 3 1 

element structure of special linear group of degree four over a finite field 
5  4  1 5 
0,2,3,4 1 
? 
element structure of special linear group of degree five over a finite field 
Related facts
 Number of conjugacy classes in general linear group of fixed degree over a finite field is polynomial function of field size
 Number of conjugacy classes in projective general linear group of fixed degree over a finite field is PORC function of field size
 Number of conjugacy classes in projective special linear group of fixed degree over a finite field is PORC function of field size