dynamically get umenu length

Simply send the message "count" to the umenu and you'll get it from the right outlet (Ex. : count 3).
When you build the content of a umenu with a folder, using "prefix thepathofthefolder", you automatically get the number of items in the folder from the right outlet ("populate 5").

Before asking questions, don't forget to have a look at the reference. Max 5 makes it so simple to access the reference (in the help file).