2 slots. DDR4-2666 SO-DIMM. The Manual will say non-accessible but you can access the memory and are allowed to upgrade without voiding the warranty as long as it is done properly with no damage. Max is 32 gigs not the 16 stated in the Manual.
See p. 41 of the Manual.
Post back with any more questions and please accept as solution if this is the info you needed.
Be alert for scammers posting fake support phone numbers and/or email addresses on the community. If you think you have received a fake HP Support message, please report it to us by clicking on "Flag Post".