Surname Overview
Surname and Naming Context
The source lists 习 (習) as a currently uncommon surname and explicitly records it among Han, Tujia, and Naxi groups. The source therefore supports its use as a surname, without limiting it to one ethnicity or one country.
Reference: 习姓的来源和历史_习的姓氏来历_姓习的名人_百家姓习
Migration and Distribution
Distribution and Migration Limits
The page records the surname in Longkou in Shandong, Lichuan in Hubei, Zhijiang in Hunan, Jinxi and other places in Jiangxi, as well as Gaoyao in Guangdong and Lushui and Longchuan in Yunnan. It also says the surname is mainly distributed in Zhejiang, Jiangxi, Fujian, and Henan. Because no statistical year is given, no population-size assessment is made.
Reference: 习姓的来源和历史_习的姓氏来历_姓习的名人_百家姓习