Hi @hugh_sun! I believe that you can specify the query parameter $count=true when fetching the OData for a particular repeat group. If you just want the count, specify $top=1 as well. Note though that counting can be an expensive operation, because it will generate and iterate over all records for the repeat group.
By the way, since you're working on repeat groups, I thought I'd mention that in the upcoming version v1.2 of ODK Central, submission OData endpoints will allow you to specify $expand=* to pull all repeat group data along with the primary data table. (See the forum discussion here.)