Regarding trills in Classical Period piano works, where the tempo is 132 beats per minute and above, in the margins, I see the trills written as 8 thirty-second notes (semidemiquavers) for the right hand.

However, at the tempo mentioned above, about 18 notes per second are required. Is this physically possible for a pianist?

If not, what is the alternative rhythm for the hand that plays the trill?

Doesn't anyone know of a documented top speed of notes per second in piano playing?

Generally speaking, in Classical literature there is no need to play measured trills. Play them freely.

"play them freely"

If there are not 2 thirty-second notes for each sixteenth note, then what would the rhythm be?

All Eddy is saying is that the "margin suggestion" to play the trill as demisemiquavers should be taken with a pinch of salt.
If you are saying that the right hand has to play this trill while the left hand has ordinary semiquavers, then I can see your problem. You may not want to play the trill as semiquavers also, but do they really need to be in a simple ratio? If so, I suppose you could try 3 trill notes for every 2 semiquavers, giving a speed ratio of 1.5 instead of 2, but ideally your brain should be able to instruct your hand to play the trill "as fast as possible" without any well-defined speed ratio.

No general advice on this issue will be applicable universally, there will always be exceptions. Would you like re-ask your question giving a reference to a specific place in a specific piece?

An irregular grouping.

I would not worry too much about how many notes you play in a trill. The important thing is that they be even, and consistent.

Just before viewing the latest replies, I downloaded a recording and heard the solution suggested: 3 RH notes vs 2 LH notes. I can't see any other way to make the trill sound even with conscious control.

Thanks to all for the excellent assistance.