Is there a reason you cannot use the list.files() function? It has a pattern argument that you can use to find files whose name ends with xlsx and it has a recursive argument that allows you to look in sub folders. If your 50 folders are all sub directories of the folder Data, you could get all of the file names with
FileNames <- list.files(path = "Data", pattern = "xlsx$", recursive = TRUE, full.names = TRUE)
You can then make a list of the same length as FileNames and read each Excel file in as an element of that list. Would something like that work for you?