Great question. Sorry I have no answer. I'm subscribing just to see if someone does have an answer.
Oddly enough, I paired my Thunderbolt via BT yesterday and I had 718 total. They were all loaded into the car system via BT and I made an outbound call via voice control with no problem. I went through some other settings since we just bought the car yesterday and when I went back to Contacts, it was blank.
I went online since I don't know why the Contacts disappeared from the Address Book and found out that there were others with the same issue - different phones including BlackBerry and iPhone were reporting the same problems but I didn't think to ask how many Contacts they had. I would think that since the HD stores 12GB of data for songs that 700+ Contacts wouldn't be a problem but there could be a limit baked into the code.
In any case, I hope someone has an answer for us on the total number possible. Thanks for asking.