I'm kind of amazed that you had conversations with your (now) husband about kids and nobody EVER asked, "hey, how many do you want?"
Like, how does that NEVER come up? And I'm being serious. I'm flabbergasted by this. Either way, all you can do is tell hm that while you want children, the burden of bearing six children isn't necessarily one that you want. Plus, do you know how expensive six children are? That's the conversation you have. Thing is, this could be an entirely moot point anyway after you have the first one or two.
Perspective.
That's what you need. Tell him that you'll see how he feels after the first child or second. Or just let him know how many you want.
Back to this perspective thing. I remember back in college I had a lot of friends who were virgins (no, seriously). We had some of the most naive and ridiculous conversations, because I had been doing the horizonal polka for a while and had perspective. Some of the women I knew swore they couldn't date a man who couldn't have sex for eight hours straight. Me and my other non-virgin male and female friends would just laugh at that assertion. But they didn't know. So they came up with ridiculous benchmarks that are only truly addressed through experience.
So don't worry about it for now. It's premature anyway. Wait until the right time comes up and say, "so do you still want six?" And make sure you do it AFTER you leave him at home with the kids and have been gone all day long.
I do think you should discuss it with him to some extent though, because there's a chance that he might stick to his cheaper by the (half) dozen guns...
My mom always wanted six kids. My father, an only child, just sort of said yeah whatever, assuming that after a few of us she wouldn't really want six.
I'm twenty and the oldest of five, and my mom keeps pushing my dad to adopt that sixth baby. He loves us, is a great dad, etc. etc. And I don't think he'd give any of us away for anything, but you know, he's still got a family too large to fit in any reasonably sized vehicle.