Enough to gain local superiority, but not too many; remember, unlike blue water combat, rivers have very defined widths, so you can only fit so many abreast.
My rough guess is 25-50, but if you wanted a precise figure, you should be thinking about how many miles of river there are on the Trident, the Godseye River, and the Rush (at least that part inside the Riverlands), divide that by one day’s sailing distance to establish patrol radii, and then multiply that by 2-3 depending on the width of the river.
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