The last startup I was involved in had two customers lined up before launch. We (well the two founders, I was technically the first employee) started by pitching their product to their contacts in the relevant industry and didn't actually launch until they had a couple of at least potential customers lined up.

The product was quite niche and quite expensive so we didn't really need many customers. So the plan was to grow largely by word of mouth and by using our network of contacts to get future leads.