How many bands does the RUTX50 aggregate at the same time?
Up to 5 bands in 4x4 MIMO configuration.
Does the RUTX50 simultaneously aggregate the three bands LTE: B1 + B3 + B20 ?
Yes, the modem is capable of such combination.
Where can this information be found?
Supported bands can be found in Qectel documentation, for example, here, or this page under Ordering tab. Carrier aggregation data, apparently does not seem to be available publicly.