First off,
@thesteelydane is absolutely right about trying to get the sound you’re hoping for with just one violinist. If you really want, say, six passes, it’ll be vastly more successful to have three players and get them to overdub once, rather than try to stack six takes.
But it’s also true that just one — a solo — if recorded right will dress out a string line to a startling degree. Just think about the samples and the positioning of the mic. Remember that even if you have front row seats at a symphony, you’re still many meters / yards from the string players as a group.
Quartet
With regard to a quartet or quintet, remember that two violins trying to play in unison can introduce intonation (tuning) problems, whereas just one (or three) on a part works fine. Accordingly, if you hired two violins, one viola, and one cello, you would most likely do just one pass (I would anyway), with each player on a different part. The quintet idea is to add either a third violin (so 3, 1, 1) or even a bass. If you have a bass, though, you will want to put up a baffle or otherwise isolate the instrument — it’s big and can overwhelm a violin with no trouble!
By contrast with violins, I have found that two violas can play in tune, though often that’s been when there’s divisi and there are actually four or six (or more violas). I don’t think I’ve ever hired only two violas on their own with nobody else playing. Plus, I’ve been lucky to have very good players who are accustomed to recording and to clicks. I also write lots of divisi so if I have two violas I’m quite likely to have them on separate lines anyway.
Left Field
Another way to tackle the sweetening thing is to try George Strezov’s team in Sofia (George of Strezov Sampling). They have excellent string players, a good studio, and might even be able to tack on a small session to the end of one of their other dates. They also can orchestrate and copy parts.
I’ve worked with them many times and they are very professional, have good microphones, good engineers, and a team that can communicate accurately and quickly in English. fourformusic.com