Even with top 25, it will be "too many" for low activity server.
Even with 50, maybe there is more than enough target in S1/2

The thing is, a hard number will NEVER work for every server. What SHOULD happen is the server self-adjusting, too many people plundering? Well, someone is gonna give up the wait and go convoy, thus more targets, thus more balance. Plunder gets easy due to lots of target? Greed will get the better of some people and they will start plundering. Eventually, there will be some sort of balance.
Oh no, the balance will not be at 3 plunder per plunderer every day, it will be somewhere around 2, maybe leaning toward 1, because even that's better than convoy.
Stop asking the "system" to do everything for you, some things are still left up to the players.