Newby, Help!!

Probably about right, depends on the initial liquid volume metric or imperial, you probably should have got 6 bottles out of it if metric as 5 litres divided by 0.75 is 6.6. Some wastage being the 0.6, so 6 bottles.

If however you made it to 1 gallon (4.55 litre) that is 6 bottles but no wastage. You will have lost some during racking etc so 5 and a half bottles would be about right.

Being a kit you cannot realistically "scale" it up a bit to have a bigger volume as you can with a fruit based one where if it says 3lb of fruit you make it 3 1/2 lbs and a little more liquid.

After racking I top up. The 1/2 bottle won't keep and should be drunk. I try to keep mine in the demijohns as long as poss before decanting into wine bottles.