The ideal situation (for efficiency) is to save a seed for each common flower/foliage combination (Spotted, Bluestar, Jalapa, Four-petal, and Citrus Reptans; Spotted, Bluestar, Jalapa, Four-petal, and Citrus Astera, etc. - 25 seeds in all), although you can make any Tier 1 through Tier 3 species starting with only 4 seeds (Spotted Reptans, Citrus Reptans, Spotted Ball Cactus, and Citrus Ball Cactus) - you'll just have to cross-pollinate more plants to get what you need.

When you're down to only one seed of a specific plant in your "starter set" you'll need to plant it and self-pollinate to make more of those seeds.