Hi,
There is one memory slot and max size of module you can use is 16GB of DDR4 2400Mhz RAM. You can purchase from your preferred brand such as Crucial, Samsung...
On page 35 of service manual you can see the steps to upgrade RAM:
http://h10032.www1.hp.com/ctg/Manual/c05527167
Example:
https://www.crucial.com/usa/en/compatible-upgrade-for/HP---Compaq/envy-x360-15-bp152wm
Hope it helps,
David
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