Below are charts indicating average chest output by Voyages at level 30 and above from the Gold Hoarders. These charts provide a range of chests-per-voyage based on the pirate who buried them.
In order to add unpredictability, Sea of Thieves implements a system to keep players from predicting patterns in voyages by periodically dropping an outlier number of chests beyond the predicted range.
Accuracy of the ranges are 80%, while the outlier drop rate is 20%. Resulting chances are not cumulative.