https://arstechnica.com/information-technology/2020/12/only-an-elite-few-solarwinds-hack-victims-received-follow-on-attacks/

Of the 18,000 organizations that downloaded a backdoored version of software from SolarWinds, the tiniest of slivers—possibly as small as 0.2 percent—received a follow-on hack that used the backdoor to install a second-stage payload. The largest populations receiving stage two were, in order, tech companies, government agencies, and think tanks/NGOs. The vast majority—80 percent—of these 40 chosen ones were located in the US.

These figures were provided in an update from Microsoft President Brad Smith. Smith also shared some insightful and sobering commentary on the significance of this almost unprecedented attack. His numbers are incomplete, since Microsoft sees only what its Windows Defender app detects. Still, Microsoft sees a lot, so any difference with actual numbers is likely a rounding error.