With meshes, we now have 2 ways to calculate prim count, old math (one prim = one prim), and new math (one prim = however many land impact prims depending on a number of variables). Any linkset linked to a mesh always uses new math (just one mesh in a linkset will force new math for the entire linkset, always). But you can choose to use new math on a linkset of traditional and sculpt prims as well.
But do that with care! There are reasons to do it, and reasons not to. Biggest reason to? In new math, a single prim can count as HALF a prim in a linkset, so one linkset of prims can be half the number of actual prims. Link six boxes and use new math? 3 land impact prims. (Note: this always rounds up. 5 prims will also be three prims, 7 prims will be 4). THIS ONLY APPLIES TO STATIC PRIMS. THEY CANNOT BE SCRIPTED OR PHYSICAL. It's a bonus modifier we get applied to "simple" prims, those that impact the server less.
However, new math is not straightforward like old math. There are 3 variables, and you have to figure out how to juggle them. Some people are just telling people to set entire linkset to Convex Hull and watch the savings: but if you don't have the right things in your linkset, your prims will count as MORE, and you may not be able to walk into your buildings anymore!
There is an easy way to find out what things will cost, and if it's worth trying (AND how to fix the not being able to walk into places problem). Click on the "More info" link to find out what your object will cost (it can take a bit to calculate though!):
Download is how complicated the object is. It calculates by number of faces and size, complicated prims have more faces. Aside from replacing prims with less complicated ones, or shrinking prims, or removing complicated prims from the linkset entirely, we can't get this number any lower. Sculpts are a MINIMUM of 1.3 Download, even tiny, even with faces thrown away in the sculpt map (that, in fact, doesn't seem to have any impact to sculpt calculation ever- sculpts seem to have a flat calculation, regardless of the sculpt). And they go up from there. Linking 2 sculpts together will become 2.6- which will count as THREE prims. So only use sculpts with new math if you can balance the cost with other prims! Sculpts and toruses and spheres and tubes- anything other than boxes and prisms (and how often do you use a prism?)- can cost you, depending on size and complexity, they can cost too much. Boxes are cool. Boxes are 0.1, even at 64mx64mx64m, hollowed (even circle hollowed) and cut! (NOT twisted. Twisted ads a very substantial amount more to it, because it requires many more faces for the twist.) So, say you have a linkset with a bunch of boxes and a sculpt? You may find you can link that together and use new math and prim count goes down. But you may not.
Physics is where you can really play with your linkset cost. Simple physics (boxes! But not as much cutting or hollowing this time!) cost very little. Complicated physics, especially hollowed or cut things, even boxes, start costing. You can reduce physics cost by making things Convex Hull, but that makes them VERY simple: you will not be able to walk though hollows or cuts, and the actual outer shape beyond that may not be what you expect (it's not straight up bounding box all the time, but sometimes it is). You can reduce physics further by setting SELECTED PRIMS IN THE LINKSET to None. The root prim CANNOT be set to None. Physics set to None works like Phantom: so you can mix Phantom and non Phantom prims in a linkset this way too! Hollowed or cut prims for doorways may have to be set to None (you can set them to Prim so long as the rest of your Physics allows for the calculations). Many sculpts you may want to set to None to take them out of the Physics calculation, as well as not having their physics get in the way. You can set all the prims in the linkset individually to your choice of the three options, and you may need to! These example prims? I need to set my physics to Convex Hull or None to bring down my cost before I use new math, or not use new math at all, as well as linking them together (in practice? The base had to be set as Convex Hull, AND all the linked prims had to be set to None, before I could get the Physics low enough to see enough savings to actually lower the prim count. They are a sculpt and 2 cut tubes, so, complicated- but those cut tubes actually don't cost all that much at this size, so I can get away with it, so long as I don't need proper physics collision).
The last variable here is Display. That will NEVER be less than half the number of prims in your linkset. In this case 3 prims, divided by 2, equals 1.5. This is an unscripted unphysical linkset, so I get the static bonus. If this is equal to the number of actual prims, it's not worth trying to use the new math, because you won't see any savings!
The largest of these three numbers will always be the one used. The land impact number is rounded by linkset, 1.5 will be 2 land impact prims, but 1.4 will round down to 1.
A lot of land impact can be saved if you are smart in how you juggle your physics- and in what you link together! But that large sculpt? You may want to unlink that to leave it using old math and make sure you select it too before you move your thing.