USE-2 and THHN: WIre Insulation Types

Source: NEC
RHW-2 in this table is listed, and it's the same as USE-2, which is an outdoor-insulated wire, recommended to connect to the panels by all professionals in the PV industry.  Here is a table which states the number of said wires inside of EMT conduit.  Solar Panels have USE-2 wires coming from their j-boxes, and the connectors are made to create water-tight seals onto that type of outdoor insulation.  

In this table, it's noted that 8 of such wires can fit inside of 1" conduit, meaning that you can have a total of 4 strings entering a 1" conduit, or else you will have to up-size to 1 1/4, which will allow you to have 13, or a maximum total of 6 strings.

USE-2 is "Underground Service Entrance" and you can buy it from nearly any source of PV supplies.  It's coated with a blood thinner which acts as a way to keep animals from eating it (it's essentially covered in animal poison) so be careful with eating after you have handled USE-2 wire.  It's also UV resistant, and because of all this, it's much thicker than THHN wire, which is why fewer wires will fit inside of conduit.  You can run this type of wire from the panels to the combiner box.  

Source:  NEC Table C.1
By comparison, THHN and THWN and THWN-2, all conductors rated to be indoors or inside conduit, can fit up to 16 of the same #10 AWG conductors in 1" conduit.  It's also less expensive than USE-2, making it the wire of choice for installers to have inside of conduit.  

Source:  NEC Table C1