Short version:- Statistically it should make no difference in the long run. ( assuming "Random Boss Drops are a % on hit" is an amount of damage done) However it is likely that you could see better result for either 5 or 20 stam hit for short periods. So it is faster and easier to use 20 stam hits. The only way to check would be to do sufficient random 5 and 20 stam hits on the same boss type and level with the same set up. and compare the result, but the number of attacks for each, required to get a statistically accurate result, could be very high.
As I have seen at least 1 drop from a 1 stam hit , statistically it would be possible to see at least 20 drops from a 20stam hit, for the same boss same player and set up, however it is very unlikely. ( Noting that there are variable relating to player /boss type among other that affect the drop rate. ) Example:- if you had 1 person flipping a coin 1 time they have a 50% chance of head. If they flipped the coin 20 times it is very unlikely to get 20 heads in a row, but it is possible. In the same way if 20 people flipped 20 coins it would be very unlike to get 20 head. The chance of it happening would be the same. If you average say 4 drops from a 20 stam hit, then with the same drop rate, you should average 1 drop for every 5 x 1 stam hits and average 1 drop every 5 stam hit.
