"If you build it, they will come" is entirely untrue in the world of SaaS. Your first ten (even your first 100) customers will each require an enormous amount of effort to find, convince, on-board and retain.

Once you've built a product that ten people are willing to pay for you can probably get to 100. If you can get to 100 you'll probably be able to get to 1,000. But those first 10-100 will be like pulling blood from a stone.

There's so much good advice embedded in https://stripe.com/atlas/guides/starting-sales about getting your first ten customers, but I particularly liked this accompanying tweet:

"We made a sale for Appointment Reminder from someone whose only way of getting data into the system was to fax it to us. Guess the cheat. If you guess “CEO signs up for HelloFax, receives the fax, and types 600 patient names and phone numbers by hand” you have good instincts." - https://twitter.com/patio11/status/922491782583037953