why not send an army of terminators back...

Maybe they have to pay export tax on each one?

I assume it's a power issue.

You can't send clothes back, so time travel must have a lot of issues.

They wouldn't be able to defend themselves (considering all the trouble they have dealing with just one) and there wouldn't be much of a movie.

Terminators should have followed after T2, three T-800's (Played by Arnie) are sent back to the 90's to kill the John and Sarah, but among the three Terminators is one reprogrammed (again by future John Connor).

Also, they can probably freely choose ANY time, so why not go to where Sarah is just a baby and just teleport right on top of it? Sarah would be dead before the terminator would even be able to print "Target Terminated: Mission Accomplished" on its visuals.

This begs the question; why did Skynet choose 1984 in the first place, when it should've realized that it didn't work, because Kyle and Sarah stopped that particular terminator?

It also begs the question, at what point does Skynet know what happened in 1984? Shouldn't it already know the 1984 events before even sending anything through, and thus knowing that it didn't work, and thus alter the time and date? Shouldn't also know to not let the humans know what time it sent the terminator to, so they can't interfere and stop it? I don't know how the humans knew the year, or if the machine simply remained in the settings Skynet had set it to before sending the terminator through, but Skynet should have done something about that.

When you start asking questions, you open the floodgates and everything becomes a 'wait a moment, why didn't Skynet..'-thing.

Saving that for the finale

Noting that almost every time travel story has some contradictions and plot weaknesses, there are a few things you could cite to explain this. First and most obvious - that it takes a holy ѕhitload of power to send even a single person or terminator back in time, which increases exponentially the further back you want to go. Also the nuclear war destroyed a great deal of humanity's military capability. That was essential for Skynet's takeover. Against individual humans with nothing heavier than small arms a T-800 or T-1000 is effectively invincible. But humans in the future got good at destroying them. That's how they won.

Large enough caliber bullets can penetrate the 800's armor and high temperature incendiaries should be able to destroy the shape shifters (since molten steel did it). If Skynet didn't know precisely when and where to strike, I mean an exact location and time, there would have to be a hunt and too many operational terminators in the past would draw the government's notice. They'd probably think initially these killers were part of some terrorist group, but after the first direct engagement would realize they were dealing with highly advanced machines. Either that or alien invaders. There'd be a massive response to crush this unknown threat.

Against well equipped soldiers the machines would probably still inflict casualties at a steep ratio. The US army has no experience dealing with these things. But we have the numbers and they would eventually be taken out. So if Skynet sends multiple terminators it would have to be quite a horde. Unless as I said it knew precisely where its target(s) would be at a given moment in time. Then the mission could be carried out quickly - and local law enforcement would be badly outclassed if they tried getting in the way. Unfortunately from the future's point of view, the Connors spent their entire lives off the grid. Skynet would have almost no information to work from.

From what they've said time travel was a late-in-the-war development, and Skynet's resources were dwindling as defeat drew near. Gathering an army of terminators and generating the amount of power needed to send them might've been well beyond its capability at that stage.