Servers show trending posts based on what server members boost, and what the accounts they follow boost. A single-member server may not have enough data to show trending posts.
However, help is at hand! If you're on a server without trends, you can follow a Hypebot account which boosts trending posts from selected other servers. For example this one is popular:
➡️ @hypebot
Combined with Mastodon's new fetch-all-replies feature, all servers can participate in trending discussions.