There are lots of little tiny IoT type things that run the arm instruction set and do it really well, but they are not in the same class as serious server motherboards.
ARM is just now barely scratching the surface of the number of pci-express 3.0 "lanes" you can get with one EPYC or threadripper socket.
There is nearly zero current ecosystem of serious server motherboards to run Linux either bare metal or with the kvm or xen hypervisors. Things with a lot of ddr4 ram, m.2 nvme SSD PCI Express slots, pci-express 3.0 x16 slots, 10 and 100 Gbps NICs (SFP+ and qsfp28 form factor).
Every arm server board I have ever seen to date is a special one off low quantity thing produced for a special purpose.
As it stands right now a manufacturer like supermicro can fit four single socket motherboards in a 2RU chassis, and populate them with something like the $750 EPYC 16-core which will run circles around anything arm based. Or a cloud scale operator can put a whole shitload of 1U size individual chassis into use with standard atx size motherboards and populate them with $275 each, ryzen 7 2700 CPUs.