In general, grouping operations are easily handled by Seq.groupBy.
If you only want to count number of items in each group, Seq.countBy is the way to go.
[ (1808,"RS");
(1974,"UE");
(1066,"UE");
(3005,"RS");
(2007,"UE");
(2012,"UE"); ]
// 'countBy' returns a sequence of pairs denoting unique keys and their numbers of occurrences
// 'snd' means that we try to pick a key as the *second* element in each tuple
|> Seq.countBy snd
// convert results back to an F# list
|> Seq.toList
// val it : (string * int) list = [("RS", 2); ("UE", 4)]