How to optimize a large object for mobile devices?

Question

Unity

Created Terrain in Blender. (whole terrain 2000x2000 Units) . what is the best way to create textures? - The object is large, so I think it will be optimal to cut it into 16 parts, but here is the resolution of the textures and how many of them, I don’t know better - there is an opinion that the more textures, the more DrawCalls , but the larger the texture size, the longer the loading, and therefore the load to the device.