In reply to nextural:
It depends...
On many routes F to AD ( and sometimes harder ) one 50m rope is fine or even preferred ( simpler and quicker ). Depending on the nature of the route ( and the experience / skills level of the party ) a half rope can be ok but some rock in the European Alps can be very rough. So, assuming some direct belays and flicking ropes around spikes whilst moving together a triple rated or even full rated single may be preferred - 'classic' weaving ridge routes and less experienced party members in particular can quickly trash ropes. Thinner ropes can also be harder to handle in gloves / cold and for the less experienced. If you do a lot of climbing on this type of route a single 50 metre workhorse of a rope will last longer.
However, some routes are not equipped with 25m abseils or regular belay points and may wander around more or have longer pitches. For example the Contamine routes on the Tacul ( AD ) can be moving together territory on much of the routes for some parties but many will pitch at least parts of them usually requiring a 60m rope or even two ropes if you think you may need to abseil / retreat.
Good question about what makes a 'classic' alpine route. I guess it is a popular route usually with a glacial approach ( Hornli Ridge is one of the exceptions ) that can also have scrambling, rock climbing, mixed and some pitched climbing that would usually all be done in big boots.
Finally, it is worth noting that guides will often take clients up routes they have done many times before and may take the minimum rope length required for that route which may be a mere 30 or 40 metres.