How much of your stock solution should you take to make the 1000 cells/mL mixture?
Oh, come on, thats simple math.
First calculate how many cells you have per ml: 50$\mu$l are $\frac{1}{20}$ of 1 ml. So multiply 125 with 20 to get the concentration per ml: $125 \times 20 = 2500 \frac{cells}{ml}$ 

If you want to know how many cells you have in total, multiply by 8 (for the 8ml you have) and you come to 20.000 cells in your 8ml solution.

When you want to know, how much of your solution you need, you calculate: $\frac{2500 \frac{cells}{ml}}{1000 cells} = \frac{1 ml}{x}$. 

This can be transformed into $x = \frac{1}{2,5} ml = 400 \mu l$.

So you need 400 $\mu$l for each ml of your final cell solution. So if you want to have 5ml with 1000 cells/ml you need $5 \times 400 \mu l = 2000 \mu l = 2 ml$ of your cell solution. To reach 5 ml add 3 ml of your solvent/media.