Cell Styles - Too Many

The maximum number of cell styles in a workbook (Excel 2007 and above) is 65,000 (or 65,476).
If you workbook contains more than this number you will see one of the following messages.

Known Bug - Unused Styles Are Copied Unexpectedly.

This was a known bug in Excel 2007 (and is still a bug in Excel 365) that is still not fixed.
When you move or copy a worksheet from one workbook to another, unused styles are copied unexpectedly.
There is a hotfix that can be applied to stop these unused styles being coped across.

HKCU\Software\Microsoft\Office\16.0\Excel\Options\  DisableCopyUnusedCustomStyles - DWORD (32 bit) - Value = 1 

Known Bug - Not working when is Excel running in separate instances

The hotfix described above only works when you are copying to and from workbooks in the same Excel instance.
If you have the "DisableMergeInstance" entry in your Excel registry options then the hotfix will not work.

HKCU\Software\Microsoft\Office\16.0\Excel\Options\  DisableMergeInstance - DWORD (32 bit) - Value = 1 

link - social.technet.microsoft.com/Forums/windows/en-US/710c7bae-eb24-4bf5-93dc-ceb453678a76/disablecopyunusedcustomstyles-not-working?forum=excel

Solution

The only way to remove a style using the User Interface is to right mouse click on an individual styles and choose delete.
If you want to remove a lot of styles you will have to use some VBA code.
This subroutine will try to remove 30 custom styles at a time (even ones that are being used).

Public Sub RemoveALLCustomStyles() 
Dim oStyle As Excel.Style 
Dim iReturn As Integer 
Dim iCount As Integer 

   iCount = 1 
   For Each oStyle In ActiveWorkbook.Styles 
      If (iCount = 30) Then 
         Exit For 
      End If 
      If (oStyle.BuiltIn = False) Then 
        iReturn = MsgBox("Do you want to delete this style:" 
                  vbCrLf & "'" & oStyle.Name & "'", vbYesNo) 
        If (iReturn = vbYes) Then 
           On Error Resume Next 
           oStyle.Delete 
           If (Err <> 0) Then 
              Debug.Print "Cannot Delete: '" & oStyle.Name & "'" 
           End If 
        End If 
      End If 
      iCount = iCount + 1 
   Next oStyle 
   Call MsgBox("All Custom Styles have been removed") 
End Sub 

This subroutine will remove all the custom styles that are not being used.
This first examines the workbook and saved a list of all the used styles into a Dictionary object.
Before running this code you will need to add a VBA reference to the Scripting.Runtime.
Warning: If you have 60,000+ styles in a workbook this code could take 20 minutes to run.

Public Sub RemoveUNUSEDCustomStyles() 
Dim oStyle As Excel.Style 
Dim oCellRange As Excel.Range 
Dim oWsh As Excel.Worksheet 

Dim sStyleName As String 
Dim iStyleCount As Long 
Dim oScriptingDictionary As New Scripting.Dictionary 
Dim aKey As Variant 

  For Each oStyle In ActiveWorkbook.Styles 
    If (oStyle.BuiltIn = False) Then 
      sStyleName = oStyle.NameLocal 
      iStyleCount = iStyleCount + 1 
      oScriptingDictionary.Add sStyleName, 0 
    End If 
  Next oStyle 

  For Each oWsh In ActiveWorkbook.Worksheets 
    If oWsh.Visible Then 
      For Each oCellRange In oWsh.UsedRange.Cells 
        If Not oCellRange.Style.BuiltIn Then 
          sStyleName = oCellRange.Style.Name 
          oScriptingDictionary.Item(sStyleName) = oScriptingDictionary.Item(sStyleName) + 1 
        End If 
      Next oCellRange 
    End If 
  Next oWsh 

  For Each aKey In oScriptingDictionary.Keys 
     If oScriptingDictionary.Item(aKey) = 0 Then 
        ActiveWorkbook.Styles(aKey).Delete 
        If Err.Number <> 0 Then 
           Err.Clear 
        End If 
        oScriptingDictionary.Remove aKey 
      End If 
   Next aKey 
End Sub 

Public Sub RemoveUNUSEDCustomStyles2() 
Dim oStyle As Excel.Style 
Dim oCellRange As Excel.Range 
Dim oWsh As Excel.Worksheet 

Dim sStyleName As String 
Dim iStyleCount As Long 
Dim oCollection As Collection 
Dim oObject As Variant 
Dim oItemExists As Variant 

  Set oCollection = New Collection 
  For Each oWsh In ActiveWorkbook.Worksheets 
    If oWsh.Visible Then 
      For Each oCellRange In oWsh.UsedRange.Cells 
        If Not oCellRange.Style.BuiltIn Then 
           sStyleName = oCellRange.Style.Name 
           oCollection.Add sStyleName 
        End If 
      Next oCellRange 
    End If 
  Next oWsh 

  For Each oStyle In ActiveWorkbook.Styles 
    If Not oStyle.BuiltIn Then 
       On Error Resume Next 
       oItemExists = oCollection.Item(oStyle.Name) 
       If (oItemExists = Empty) Then 
         oStyle.Delete 
       End If 
    End If 
  Next oStyle 
End Sub 

Windows App - xlStylesTool

This can be found in the Windows App Store