Ash here from the Shopify team. Welcome to the Shopify Community!
Regardless of how many products you plan to sell, a paid plan is required in order to use Shopify as your ecommerce solution. Shopify is free to test out for 14 days, but at the end of the free trial you'll need to select a plan in order to continue with your account. The features and pricing of each plan are available for you to explore here, and if you're looking for the least expensive way to use Shopify, you may also be interested in learning more about the Shopify Lite plan here. There's no online storefront included with the Shopify Lite plan, but it's a great solution for merchants who want to sell in person, sell on Facebook, or who already have an existing website that lacks an ecommerce solution.
I'd love to hear more about the product you plan to sell! Feel free to reply back here if you want to discuss your business idea further, or if you need any assistance getting started with Shopify!