Can Shimano Claris use 11 speed cassette? I'm sort-of new to road biking. I'm planing to upgrade my bike and group set slowly (since I'm a broke high school student). Right now I'm running Shimano Claris group set (except I have a R8000 Ultegra rear derailleur) because I'm planning to upgrade from 8 speed to 11 speed, however I don't have other shifter levers. 

Can I use Shimano Claris levers for 11 speed?
No, that will not work. The latest generation R2000 Claris groupset and it's predecessors are all 8 speed systems, so you can't select 11 sprockets on the cassette.

If you are thinking that you can use the 8 of the 11 sprockets, that won't work either.

A quick explanation of the differences between different 'speed' groupsets: the sprocket spacing on 8, 9, 11 and 11 speed systems gets progressively closer, the sprockets narrower and the external width of chains narrower also. Proper positioning of the derailleur over each sprocket is controlled by the incremental length of cable pulled by the shifter and the actuation ratio of the derailleur (the ratio of cable length pulled to lateral movement of the cage).

So, an 8 speed shifter, even with a 11 speed derailler, won't position the cage over the sprockets on an 11 speed cassette properly.

Additionally, the 11 speed Ultegra derailleur isn't compatible with the 8 speed Claris group. Shimano 11 speed road derailleurs have a different actuation ratio than 10 speed and below. Using an 11 speed derailleur with 8 speed shifters and cassette will give you trouble with indexing all the sprockets properly.

Shimano publishes extensive specifications and compatibility information you can consult to figure out what components work with each other https://productinfo.shimano.com/#/.