Depends on what type of curator you are.
Spotify Playlisters
Your genre cap scales with how many qualifying playlists you have. Each playlist that's hit 1,000+ followers and is actively used unlocks 15 more genre slots. So one qualifying playlist gets you up to 15 genres, two gets you up to 30, three gets you up to 45, and so on, up to your overall cap.
Other curator types
Bloggers, YouTubers, radio shows, and other non-playlist curators don't have the per-playlist scaling. They get the flat overall cap from the start.
What's the overall cap?
45 genres for most curators. Some legacy accounts and lower-priced tiers can have slightly higher caps, but 45 is the typical ceiling. The thinking: the more genres you take, the less focused your taste signal is, and the worse the experience for submitters trying to find a good match.
Worth noting
Curators with fewer, more-relevant genres get a quality-score boost. So even if you qualify for the maximum, being deliberate about what you enable is usually a better strategy than maxing out.