Scion vigor, rootstock vigor, soil type, irrigation, and training system all effect tree spacing. I would suggest you look at this tree spacing calculator and play around with the different settings to get a better feel for how all of them interact.
Note: the calculator is setup for North/South rows- with East/West rows you need a bit wider between row spacing due to how sunlight hits the trees.
http://fruitadvisor.info/tfruit/clements/appletreespacing.htm
The calculator doesn’t cover some of the newer Genveva rootstocks so use this chart to convert a newer rootstock like G890 to older rootstock sizes G30/M26/M7 etc.
A list of scion vigor for many apples-
The problem with trees labelled semi-dwarf is that vendors are very loose with that term. The rootstock could be M26 or M111 or something else. My neighbor had a Golden Delicious on “semi-dwarf rootstock”. The tree was 30 feet tall and 30 feet wide. And Stark Brothers has sold semi-dwarf trees on M111 in the past.
If you can get trees labelled with the actual rootstock M7,G890, etc. This will make planning spacing much easier and much more accurate. Also if you have problems with the tree it makes troubleshooting easier.
The training system makes a big difference on how far trees have to be spaced apart. For example, using the tall spindle system, my Goldrush on G890 (an M7 sized rootstock) is supposed to be spaced 4 feet apart in row according to the calculator. For an unsupported tree trained to central leader system the spacing in row is supposed to be 16 feet. This is a huge difference.
You might also look at this thread where apple tree spacing is discussed in detail.