Always best to install RAM in matched pairs. One stick only runs in single-channel mode, while a matched pair runs in dual-channel mode. Dual-channel is somewhat faster and more efficient than single-channel mode. Never good to mix RAM from different manufacturers in the same bank (= pair of slots with same color retention clips)  If you get 2x 8 GB now, you could add 2nd pair of 2x 8 GB in the future for 32 GB total. Or you could add 2x 16 GB for 48 GB total. Dell doesn't usually list 48 GB as a compatible RAM config in their specs, but that probably means they just haven't validated 48 GB.  Otherwise, get a pair of 2x 16 GB now and still have 2 slots for even more RAM in the future. You don't have to buy the 2nd pair from Dell, just as long as the 2nd pair is compatible with the original pair (and the motherboard). Many users around here buy RAM for Dell PCs from Crucial because they guarantee it works or they take it back. 
I have a question regarding the combination of RAM I should get now if my intent is to upgrade in the future.  I would like no less than 16 gigs now.  I have looked at the 8950 specs and am a bit confused as to how I should have it configured.  Should I go with a pair of 8 gig sticks now, or go with one 16 gig stick, if my intent is to use the RAM that comes in the computer now with the intent of upgrading it to 32 total gigs later on?  If one stick is put in instead of two, does that limit the ability to use the adjacent slot, or does DDR5 memory not need to be installed in pairs since for 8 and 16 gig configurations one stick can be used instead of two? I want to avoid having to remove the memory in the future of the 16 I want to buy now so I don’t need to “buy double” in the future to have it running 32 gigs.  If I understand correctly, the only 32 gig configuration listed is for a pair of 16 gig sticks.  My concern is that now If I order the memory with one 16 gig stick I won’t be able to add another single 16 gig stick based on single vs dual channel, or is that not an issue?
XPS 8950, future RAM upgrade