Most x99 mATX boards have 4 ram slots, like that Gigabyte X99M, which has a maximum of 32GB of ram anyway.
There’s nothing inherently worse about having a mATX computer vs a full ATX, although with X99 there’s a lot less options for motherboards in that size. I’m using a mATX x99 build right now. And Malachi93 asked for it to be portable.
Although if I wanted to have a rig to bring to clients, I’d probably just go ahead and have 2 builds, a beastly x99 for working in the office, and a super portable ITX build that you could literally throw into a backpack. And you could setup the portable PC as a mini render box, for lightmass or anything else.