Dilutions confused me here. . .

For this typical procedure, I make 5ng/uL (nanograms per microliter) of DNA from a stock solution.

From the stock I have to dilute it 1:10 (10uL DNA and 90 buffer) to make it 50ng/uL

Then dilute that again 1:10 (this time 10uL DNA, 70 buffer and 20 tracking dye) to make it 5 ng/uL.

I then use this solution to put as samples on a gel for electrophoresis, and each sample has to be 50ng/uL (so I pipette 10uL from the 5ng/uL dilution.

The problem that confuses me is that now I need to do 12 samples. The 5ng/uL solution is only good for 10 (Because I use up 10uL for each sample and the dilution is 100uL) I am confused as to how to accommodate a slightly larger dilution without messing up the concentration. The more I thought about it, the more confused I got. I was thinking of simply adding 20 more uL of buffer into the 5ng/uL dilution, but wouldn’t that dilute the DNA more? Yet I’ve done things like that before when I looked back at older experiments and now I’m confused as to why the concentration of DNA doesn’t get more dilute even though I add more dye here and there or more buffer here and there.