The history is limited to 10 examples, because those are the answers that have not yet been sent to the server. They're kept on the client so the annotator can easily hit undo, and without requiring Prodigy to reconcile two annotations later (the original one and the edited answer). It also means that datasets can be "append-only" (see here for some background on this).
We could introduce a setting to change the number of examples kept in the history, but this would mean that you'd delay sending the answers, which isn't that useful. So during a single annotation session, you can currently only undo the most recent 10 examples.