Anything from 50 watts to 150 watts, with the best results being in the 75w to 100w range.
The power rating of a speakers isn't a requirement, it is a rough indication of the power limits of the speaker. A good rule of thumb is an amp that is equal to the continuous power rating or just under it.
I'm often fond of saying - 'It is never overpowered or underpowered amps that blow speakers; it is always they guy running the volume control.'
Likely as you move over 100 watts, money will limit your power more than anything. Few can afford amps over 100 watts per channel, and that is a working maximum rating for many many speakers.
In short, don't worry about it, find an amp you like that sounds good, and you can afford, and be happy with it. Though, in general, the more power the better, on the assumption you aren't compromising quality. Somewhere in that range, you have to find the amp the strikes the best compromise of quality, quantity, and price for you.
But relative to power, more than anything, your budget and your demand for quality are going to be the limiting factors in selecting an amp.
Regards