While this answer can depend on your keyword competition and product niche demand, finding one or two keywords that are relatively low competition, high search volume to target is ideal. Once you dominate those keywords (most often longtail keywords), you can branch into the more competitive keywords with more revenue and positioning power on Amazon. Spending your marketing dollars as efficiently as possible can make a tremendous difference. If you have a big budget, then there would be nothing holding you back from targeting the large keyword phrases from the get-go. But if your budget is limited, targeting multiple keyword phrases that do not require as many units to be given away would be more ideal.