I'd consider starting with eight designs or nine designs. That way you can have your shirts displayed in a neat grid (4x2, 2x4 or 3x3) on the page with no empty slots. Eight also just seems like a good number to give plenty of choice without being too over the top. That said that was an arbitary number choice on my behalf, and five sounds good too. Or six - 3x2 of current designs, then 3x1 of "Coming Soon".
As for your "Coming Soon" / Register Interest idea - it sounds like something worth considering. I don't know if people actually use those features, but in my opinion there's nothing wrong with them. So long as you make it clear people are under no obligation if they register interest (don't want to put people off) then I don't see why not.
If you're using it to gauge interest though, "Coming Soon..." is a little inaccurate, as some designs may not make the cut. Also, while "Coming Soon" is a fine concept in theory, you need to be very careful when using it. Two examples:
1) Recently a large company had a well known product recall in Australia (not bothering with details as it's not relevant to t-shirts). They then registered a website to announce their return, and spent a lot of money advertising this website on TV. On this website is a "Coming Soon" banner for some of the content. My immediate reaction was one of contempt - in their case they are a very large company, and their product was off the shelves for several weeks. They have the time and the money to have their product complete before launching. Why launch half a product?
Now obviously the larger the company, the more this is a problem. For a small company it's a different situation, but you still have to be wary. Some people perceive "Coming Soon" as admitting you only have half a product, or as being unprepared to launch. Others view it as an insight behind the scenes, an exciting chance to know what's happening next.
2) We've probably all been to a website that said "Re-launch soon!" or "Coming Soon" or "Check back for updates!". Maybe we've come back a week later, only to find it hasn't been updated. "Okay" you might say to yourself "Maybe they put that up only just before I was last there. I'll give them another week." You come back a week later - nothing. Three or four weeks later you check back again - still nothing. It's now been over a month. This time you don't check back again, in fact you write off the company as too lazy or unorganised or not caring enough about their web customers.
Obviously you can't afford to do that - if you say something will be Coming Soon you need to be positive you can make it happen within a reasonable timeframe. Better to announce something as being up without any advance warning, than to tell people it's coming and then disappoint them. If you have the content ready to post and are just holding back waiting for the right time, this is not a problem.
Two other things to consider: You want to look reasonably large and professional, regardless of what the reality may or may not be. Secondly, how many designs you have depends a lot on your printing methods. If you use a print on demand system, you might as well offer a large range of designs from the start. If you are needing to invest capital in printing in advance then you need to consider your budget before considering how many designs to print.