Every August ish. They follow a school type schedule. 2020 numbers are good for about 4 months of 2020 and 8 months of 2021 and the trend continues.
With that said naca uses current numbers for determining purchase location but is still using 2019 numbers for loans. They are a private company and thus not required raise the loan limit. There have been questions regarding this every few weeks for the last 18 months and nobody gets an answer other than the government allows private mortgage counseling services to set the loan limit below the federal limit.