I categorically disagree. The players can't know this, as the GM doesn't even know this.
However the characters can learn how large of a force they might be engaging, and use that to pace themselves. GM's can make this information available and reliable to players who seek this out.
What I mean by making it available includes throwing the information at them enough times that if it's missing they notice. So freely give the ranger "looks like a band of between 20 to 30 orcs based on the footprints and camp left behind". Or whatever.
And try and avoid screwing them over for using that info, I.e. "this isn't even my final form" can be bullshit unless you foreshadow it.
And now you have the characters making in game plans based on in game info, and interacting more with the world to make those plans. That seems a lot better than telling the players how many encounters you planned.