In general, the amount of simultaneous conversions is determined by the amount of threads of your processor.
The amount of threads in processor is determined by the amount of cores - it is either equal to the amount of cores or equal to the amount of cores multiplied by 2 (in certain high-end Intel i5 and i7 processors).
The easiest way to check the amount of threads of your processor is to open Task Manager (press Ctrl+Shift+Esc or Ctrl+Alt+Del in Windows XP) and go to Performance tab - the amount of processor load graphs there is the amount of threads, and thus the amount of files you can convert at once.
However, there is also an exception to this principle - if you are converting video file encoded with H.264 codec into any format or converting any video into format which uses H.264 codec, then 4 processor threads will be used for such conversion.
Example:
You have Intel i5 4-core processor or higher with each core supporting 2 threads, thus 8 threads in total. You are converting 5 video files - 4 of these files are AVI videos encoded with MPEG-4 codec and 1 file is AVI video encoded with H.264 codec. Output format you select is WMV, this format does not utilize H.264 codec. 4 threads of your processor shall be used for conversion of 4 AVI files with MPEG-4 codec and the remaining 4 threads shall be used for a single conversion of AVI file with H.264 codec.