It's far less common than linux oses... In any type of servers, including data storages. It is THE major target because it is a bad OS, nowadays primarily used by companies that haven't a good IT for the typical file shares used by tech illiterates easily victims of social engineering attacks. It's a explosive combination that results in that stat... Practically 100 % of successful ransomware attacks on servers is on windows servers, despite overall being much less used than competitors