As per MySQL documentation GET_LOCK() you cannot hold more than one lock per connection.
As it says
If you have a lock obtained with GET_LOCK(), it is released when you execute RELEASE_LOCK(), execute a new GET_LOCK(), or your connection terminates (either normally or abnormally).
So essentially, it depends on No.Of connection. I would say the equation would
No.of GET_LOCK handled = NO.Of Connections handled
I see there is a bug logged where people suggested to have concurrent lock per connection. See here http://bugs.mysql.com/bug.php?id=1118