The answer depends on how what type of computer you are using.
Windows: The conversions occur sequentially. So one file must complete its conversion before another can begin.
Mac: The number of conversions that can convert simultaneously is limited to the number of cores on your processor. So if you have 8 cores, you can process 8 videos at once.
Tip: If you are running into problems while converting multiple files, try reducing the number of files that you are trying to convert.