You need a certain concentration of your amplification product (which you have to check with your sequencing facility) for sequencing. Since you can always increase the volume simply by adding PCR buffer to the PCR product before purification the volume per se is irrelevant.

I'm not sure about genomic DNA. I've carried out PCR from cDNA with a final volume of 15 ul. I used 4 ul for electrophoresis, and the balance (~10 ul) was used for purification. That was enough for sequencing, but of course the bands in the gel was quite bright (indicating high PCR product).

1. I know what you meant. You want to save DNA template (genomic DNA) for many experiment, so you want to use as few as possible of your DNA template for PCR, and yet the DNA concentration of your PCR product is enough for DNA sequencing. You should pay attention to the DNA concentration of the PCR product, not the 'volume' of the PCR product. Because the same volume of PCR products can have high amount of DNA molecules or low amount of them depending on PCR conditions, such as (1) the amount of DNA template you use, (2) PCR cycles, etc.

2. In your case, use less amount of DNA template, you can increase a few PCR cycles and use a High-Fidelity DNA polymerase to minimize the mutation rate. For sequencing, usually a regular PCR product should be be enough. Also, remember, when your purify your PCR product using a column you will lose some DNA.

Volume is not crucial. But you need the specified concentration of DNA for sequencing.