Why could only one Terminator be brought back at a time?

Question

In the movie Terminator why was only one Terminator allowed to be brought back at a time?

Answer 1

Based on the script of the original movie, it wasn't question of only being allowed to bring only one Terminator back. There simply wasn't time for Skynet to send any more, as the humans had stormed the compound where the time travel device was kept. It had just enough time to send the first Terminator back before the humans found the device and sent Kyle Reese after it into the past.

From the script:

       REESE
                  ...it had no choice.
                  The defensive grid was
                  smashed. We'd taken the
                  mainframes...
                  We'd won.  Taking out
                  Connor then would make no
                  difference.  Skynet had to
                  wipe out his entire exist-
                  ence.  We captured the lab
                  complex.  Found the...what-
                  ever it was called...the
                  time-displacement equipment.
                  The Terminator had already
                  gone through.  They sent two
                  of us to intercept, then
                  zeroed the whole place.
                  Sumner didn't make it.

Of course, the timeline gets a lot more confusing in the sequels and I'm not sure if this explanation still holds up if you take them into account.