how do i code parent selection automatically?

i want to make acoding on ga GA parent selection to using crossover rate

RS is the parent that i have already sorted the fist collumn is the numbering for the result, 2nd collumn is the value and 3rd untill 14th collumn is the result arrangement

RS =

1       31868          11           4           7           5          12           3           9           1         2           8           6          10
2       35816           8          10          11           6           9           4           5           1         7           3           2          12
3       39008          10           4           5           2           9           7           3          11         6          12           1           8
4       39674           4           9           2           6           5          11          10          12         1           7           8           3
5       42110           3           5           9          11           7           4           2           8         1          10          12           6
6       45418           6           3          11           7           8           5           1           2         4           9          10          12
7       47514           8           9          12           2          10           3           7           4         1          11           5           6
8       50936           8           7          12           2           5           9           6           4         3          10           1          11
9       53966           7           6          12           3          10           8          11           5         4           1           2           9
10      57422           5           2           6           4           9           8          12          11         7           1          10           3

after the sorting i want to choose parent to make a crossover using 0.4 as a crossover persentage

%% parent selection 
cp=0.4;              % Crossover Percentage
np=2*round(cp*r/2);  % Number of Offsprings (Parents)

from the calcuation i need to chhose 4 out of 10 parent so i want to choose parent 1 and 2 then 9 and 10

then i do it like this

 p1 = RS(1,3:end)
 p2 = RS(2,3:end)
 p3 = RS(9,3:end)
 p4 = RS(10,3:end)

however when i do it like this, if i change the crossover persentage to other number i need to do it manually..

so how can i make it automatically choose whatever i change the crossover rate?

for example if i change the crossover rate to 0.6 so i need to choose 6 parent

when i run the coding will automatically choose 3 parent from top and 3 parent from bellow..