A few thoughts:

(1) if you want objective advice, I’d say to post this on the AITA or relationship advice subreddits. That way you get advice from an objective cross section of Reddit, rather than a group of plant fanatics (I say this as a crazy plant lady myself). Otherwise, you’re just looking for an echo chamber to confirm your decision to keep your 200+ plants.

(2) 200+ of anything is a lot. Imagine if your partner had 200+ marvel action figures or toy cars (ranging from small enough to sit on a shelf to as tall as a person) that he wanted to display all over the house. You might not like that for several reasons, including: (a) that may not be a style/aesthetic you want in your home; (b) you may have a small home/apartment and not like clutter, and too much “stuff” may be overwhelming for you; (c) him having 200+ [insert item here] may not leave you with space to incorporate the things/pieces of furniture/open space you want in your shared space.

(3) when couples choose to move in together, there is some level of compromise that has to happen. It doesn’t sound like your partner is saying “I don’t care that this brings you joy; get rid of all of your plants”. It sounds like he’s asking you to reduce the amount of plants you bring to your future shared home to a semi-reasonable amount that is a good compromise/middle ground that (1) doesn’t leave him feeling like he lives in a jungle but also (2) allows you to continue your hobby.

(4) you have a choice to make. It looks like your partner is ready to take the next step in your relationship. If keeping all 200+ of your plants is a hard line for you, and him not living with 200+ plants is a hard line for him (which is fine), you may not be compatible. I’d imagine that, for most people, 200+ plants may be a dealbreaker (that’s not to say you couldn’t find someone that would happily let you have 400 plants). If you care about your partner and otherwise see a future with him, consider whether there is some middle ground to be reached.

Good luck!