At this stage, it's important to understand why your customer base hasn't grown yet. Two scenarios are possible:
Awareness: your future customers simply don't know who you are - yet. When this is the case, customers who do find your product are "sticky." They tend to be enthusiastic users who come back and want more of what you're offering. This business will scale directly with marketing traction - the main question is optimizing the cost per customer acquisition against income for you to scale your business. If this is your struggle, try our right growth tool and find tactics to help grow awareness of your business.
Your product doesn't quite fit the market: You may have delighted your initial customer, but your conversion funnel for new customers is awful, or they just don't stay after trying your product. Your first customer may have had such specific custom requirements that just don't translate to a broader market, or perhaps your product isn't yet stable enough to appeal to the next wave of customers. Either way, you have some learning and development work to do.
Leverage your professional network or find your customers where they already engage like at trade shows, reddit forums, meetups, LinkedIn. It will take some trial and error, but keep trying new places until you get the feedback you need. Have open ended conversations, and focus on learning about them. Don't try to sell or pitch anything until you've learned more about what they need. Use their honest feedback to adjust your product and its features.
If you haven't already, put your product out there and start marketing. Measure engagement throughout the customer journey to understand which stage of your funnel just isn't working. Is it awareness (which advertisements could solve)? Are your customers confused by your offering? Do they need more support than you provide? Use analytics throughout the customer lifecycle to truly understand where you're loosing your customers - then fix it.
Learn as much as you can from both the customers you win and the customers you lose. Conduct small-scale experiments (like promoting potential new features) to learn more about what resonates better with your customers. You can also experiment with other pivots as well - perhaps there is another customer segment or business model that will fit your offering better. Conduct experiments and measure results to understand if a pivot - and which one - is right for you.