Officially, 12 gigs. See the Service Manual here:
Service Manual link
See page 47. The Manuals on the Pavilion 15 are often a bit hard to figure out, not to say they are sometimes a little misleading. Some models come configured with either one or two memory slots so the first thing I would do is open the memory bay and see if there are for sure 2 slots. Your laptop came with a single 8 gig DDR3-1600 module so the issue is whether you have an open slot. If so, the specs say you can add a 4 gig module of the same speed and type. Others have reported success with adding an 8 gig module to make a total of 16 gigs. Pay attention to how many black blocks are on the side of the module you already have. Likely 8 which makes it low density. Get the same kind whether a 4 gig or 8 gig module.
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