I'm not usually one to keep a lot of messages at once, but I decided to test this today. I got up to 96 Inbox 37 sent before I went loco and had to do an erase all. Prior to doing the erase all I got the "Inbox 91% full" warning message, so I'm gonna say that you probably won't get 300, but it's also not as limited as my LG's were, they would max at 50 In/50 Sent.
I think it just depends on the hardware... the VZW os can only work with what the actual phone manufacturer builds in. I have a chocolate but rarely use it for messaging (xv6700 for that) and I have 100 sent/87 inbox. No warnings. However both the V (9800) and enV (9900) have warnings that are annoying for a phone with a full keyboard (with such a heavy messaging device, you wouldn't expect to empty the box every other day.. I hated that). So it's not just the os that's causing it. I've seen a razr on VZW with like 400+ messages bunched up inside the inbox.