Have you gotten away with? I know v vocal had its limits before you had to bounce to clips. Is melodyne similar in any way like this? I've done a dozen small phrases in a five minute song and all seems fine. I've also copied tracks and made harmony vocals for all choruses. This normal use?

I've upgraded Melodyne Essential to Melodyne Studio, and found such limit.  It seems to take as many instances as your processor/ram/HDD can handle.  I've not yet hit that limit.
 
DiDi
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Sonar Producer X3e
Windows 8 Pro x64
Intel i7-3770K at 3.5/3.9GHz
16GB 1600Mhz Dual channel DDR3 ram
RAID level-0 with 2 Seagate 7200RPM hybrid drives
MPEX-1394-B3 Firewire 800x2 400x1
MOTU Traveler  Firewire audio interface
MOTU Ultralite Firewire audio interface

I've basically have the same specs as you. I noticed I had to go to my asio settings and change to 1024 from 64 which I use when tracking

Btw melodyne said I should make the asio change So I guess I don't monitor those tracks when overdubbing?

The increased latency shouldn't affect your ability to overdub.  Sonar compensates based on the ASIO reported latency to keep the input time-aligned.  The only issue is if you monitor live input through Sonar rather than through the audio device. More that 15ms total round trip latency starts to becomes noticeably distracting.
If you need these tracks to run at low latency, you can consider freezing or bouncing the track
 
BTW, I always mix/master at higher latency, to allow higher plug-in  and track counts.
 
DiDi

I see the question about several instances of Melodyne irrelevant because I always bounce after doing the edits and then delete Melodyne from the project.
I'm not the only one who has experienced problems with leaving an active Melodyne in the project. Bouncing immediately is, if not quite a concencus, but a very commonly recommended procedure.

FWIW, I've never found it necessary to bounce V Vocal clips and I've had in excess of 50 live, unbounced clips in a single project.
 
I achieved this on my old XP system running 4Gb of ram.
 
I also tend to favour VV over Melodyne, mainly because I find  VV gives me more control regarding pitch, formant & timing