Amazon narrows Romance down by subcategory because it is huge. Look to the menu on the left to see. I'd say focus on what you like and write romance that caters to that. Answer a few questions to "niche" yourself: What age range are you focused on (in terms of characters)? Small town or big city? Contemporary or historical? What world does this inhabit (royalty? sports? business? college?)? What is your hero like: Harsh (dark, mafia), witty (romantic comedy), mean (YA bully), arrogant (alpha). the more you focus the more dedicated your audience will be.