How do I decide between multiple tilemaps or one?

Hi guys,

I have a game where there are 25 screens of maps (the user only sees one screen at a time but they're all connected) and each screen is 20x15 tiles. I started building it with a tilemap then copy/pasted it but now just wondering what's the best approach.

A single tilemap for the entire game (5 screens wide by 5 screens high = 100 tiles by 60 tiles) or one tilemap per screen? I know the tilesets are optimized but does the system optimize across multiple tilemaps in a layout? Does it matter?

Thanks!