That is maximum power - 20 watts total - would be my expectation.
Which has nothing to do with the amp you buy.
This is really a very old speaker, and specifications were likely extremely conservative. But, unless you start playing things at full volume, you won't be handing it 50 watts from a 50 watt amplifier, but a much lower level.
Generally speaking, people damage speakers more often by underpowering them vs. overpowering them. So, getting a 50 or 100 watt amplifier won't hurt them unless you crank the volume all the way up.