Missing two is not the end of the world.
A full ATX motherboard will have places for 9.
For the moment, Just omit the ones that provide the least support.
You should be able to get by OK like this. In the old days you would only get 4 or 5 metal standoffs, and maybe a few plastic ones that would snap in. As long as your computer is just setting still it doesn't take much to hold the motherboard in place. If you were a builder and you were going to send the computer by UPS halfway around the world it you might have more reason to worry about it.