Hi Kaiqi, thanks for writing to us on AskGuru.
With these ethnic quotas, there are limits set on the total percentage of a block or neighbourhood that may be occupied by a certain ethnicity. The quotas are updated on the 1st of every month and you can check them out on HDB's website here: https://services2.hdb.gov.sg/webapp/BB29ETHN/BB29STREET
Learn more about how the ethnic quota can affect your selling/buying ability: https://www.propertyguru.com.sg/property-guides/how-the-ethnic-quota-can-affect-your-sellingbuying-ability-6747
Hope this answers your question.