Using below code you can get Repeater's Row/Item Count:
in below code "Container.ItemIndex" is used to get current row number and
"(string)rpttags.DataSource).Length" is used to get total row/item count.
in above line my datasource for repeater was a string array so i used string as datasourcetype and used length to get its count in same way you can get for all other types
<%# (Container.ItemIndex < ((yourDataSourceDataType)rpttags.DataSource).Length - 1?",":"") %>