there is no mathematical relationship to determine a suitable overlap, but the tile number determines the tile size and the overlap should be considering the tile size.
For example, if your raster has 8500 columns and 6300 rows. If you select 10x10 tiles, each of the tiles has a size of 850 x 630 pixels. an overlap of 200 is fine.
But if your raster has 2300x1400 pixels and you select 10x10 tiles, the size of the tiles (230x140 pixels) does not allow an overlap of 200 any longer, because the overlap would be larger than the number of rows. It then makes more sense to reduce the number of tiles (e.g. 2x2 > 1150x1200) and stick withan overlap of 200 .
The overall idea behind the overlap is to seamlessly merge the unwrapped tiles so that no “chessboard pattern” is produced in the unwrapped raster.