Is there a "magic number" of stores to have prior to launch?

I have started a service that allows liquor stores to show their products and pricing on my website, giving them full control of their profile, etc. The idea is that users will go on the site and search through the inventory that they are showing online and then go into the store and make the purchase. The website is still closed to the public because I want to populate the website with stores before launch. It has been difficult signing up stores pre-launch but my question is this:

With your experience in other industries with a similar concept, was there a magic number of stores/users/etc that was needed prior to launch?

I do not want to have too few stores before I launch because then users will not see the value in using the website and not come back. However, signing up many stores prelaunch will take a long time and possibly angry stores that I had already signed up. Also, I think stores will be more willing to use a service that is already up and running. I am planning to start the service in Southern California and grow from there.

Why don't you do a soft launch?
Make it public, so that the liquor stores you're trying to sell it to can see that it really works, but don't advertise.
Then you can "launch" whenever you're ready.

There is no benefit to the store while the site is still closed to the public. They want to to launch as soon as possible. They need the public to find their site to drive some sales.

You need to determine how much money you need to provide the service for one year. This will pay for all the costs going forward. Any money you have already spent is a sunk cost and you can ignore it. If it takes $1000 per month, then how many stores do you need to break even?

Assuming that you don't have enough signed up at this time, are you willing to lose money for the next year? Of course if you are losing money you won't be making a living either. If you are happy with the calculation, then go forward. Any time you delay before launch will just make the stores that have registered upset.

I have assumed that your hardware and software are ready to go, and that you have met all legal requirements for your jurisdiction.

Difficult to determine how many "stores" would be acceptable - how do you break down your territories? Are we talking town / county / metro / state level?

As for number of site visitors, how many prospective users have you interviewed? What is the overriding value proposition your site offers vs something like wine.com ? Or a local store with a newsletter with weekly deals?

From an outsiders perspective (and one without liquor retail experience, so take it with a grain of salt) I'm not so sure I would want to be in a portal that would expose my prospective customer base to a competitor and start a pricing war. What value-add are you providing the vendor beyond simple website exposure? Beyond what google local business center / coupons can provide?

If you can provide the vendor a strong value proposition to target additional upsell opportunities to their existing client base, then you don't need any pre-existing site visitors. Just a thought.