While aggregating two LTE carriers in Europe exists but is still not very widespread due to the availability of 20 MHz single carriers there is talk in the industry about aggregating 3 carriers. There is still some room in the specs for the moment as the maximum number of aggregated carriers that can be accommodated so far is 5. But eventually, carriers might want to go beyond that as well so 3GPP is gearing up to work on a solution to eventually combine up to 32 carriers.
The work item description can be found in document RP-142286 presented not long ago in TSG RAN#66 in December 2014. At first I thought extending CA beyond 5 carriers might be straight forward by introducing a couple of additional information elements and extensions. But that's a bit too short sighted as the current solution puts all uplink transmissions including channel feedback on the primary cell (i.e. the primary carrier). So as more and more devices become carrier aggregation capable there's more and more uplink traffic in the PCell which increases as more and more carriers are aggregated. Therefore the model does not scale well and uplink traffic and feedback at some point needs to be distributed over several carriers if more and more of them are combined.
From an overall conceptual point if find an aggregation of up to 32 carriers quite interesting. Before the aggregation of 2 carriers made it into chipsets many people were saying that this is going to be difficult as it would increase complexity and hardware cost significantly. Fast forward to 2015 and the aggregation of 2 carriers is in the wild and built into many devices. Obviously the aggregation 32 carriers is yet again another beast. And then again who would have predicted just two years ago that we would see mass market devices that support 20 LTE bands?