According to Kyle Reese, "Skynet's defense grid was smashed; we'd won." meaning that John Connor's forces had defeated Skynet & the supercomputer's defeat was inevitable. Sending a Terminator back was a last ditch effort on Skynet's part to try to erase the leader of the Human Resistance from history. It is unknown whether Skynet had the time, resources, or troops in general, to send back more than one Terminator. However, unused parts of the script from Terminator 2: Judgment Day detailed a backstory in which the Resistance enters the Skynet facility, which contains the Time Displacement Device. They also find a cold storage full of inactive T-800 model 101 Terminators, of which only one has been sent, indicating that lack of infiltration units was not the issue. Most likely, the process of time travel takes a lot of time and energy, only allowing for one person or object to be sent at a time; if more could be sent at a time, the Resistance would most likely have sent more people back than just Kyle Reese. We can also assume that time-travel is pretty much Skynet's last resort, something they would only do on the brink of destruction. The reason for this may be that time-travel always carries a danger of making unwanted changes to the past, which may lead to unpredictable, but very negative consequences in the future. So Skynet probably did not start sending back Terminators until their very last moments; they had time to send just two (one T-800 and the T-1000) and got interrupted before they could send a third one. After sending Kyle back, Connor and his team destroyed the Time Displacement Device to insure that neither Skynet or anyone else could use it again.
However, assuming they were able to send back a few Terminators or a whole army, they are likely to run into significant resistance from present day human armies pretty quickly, which number in the millions and are backed by intact governments with fully functional industrial bases. Not to mention, while Terminators are virtually impervious to bullets, they don't fare so well against explosives. So it wouldn't take long for a modern army to discover this. It therefore makes sense that Skynet sends Terminators back one at a time, but to different time periods: it would minimize the risk of detection, while at the same time doubling the chances that one of them will be successful.
We're also talking about what is essentially a stealth mission: the Terminator is, as described by Reese, "an infiltration unit", designed to be mostly undetectable. Skynet obviously recognized this as a mission parameter and probably wanted the Terminator to complete its mission as "quietly" as possible without anyone finding out about the technology used to create it, or unnecessarily changing the future. So sending one Terminator (in this movie alone, we know there were more sent back) for the sake of maintaining the original timeline as much as possible, was much more prudent. Think about it this way: if there were, say, a dozen or more or a whole army of Terminators marching around, humans would find out about their future much faster, and Skynet needs to protect its own existence. The whole theme of the future war is predicated on the idea that only Sarah, John and John's future wife know about it.