See http://word.mvps.org/FAQs/MacrosVBA/GetNoOfReplacements.htm.

It can be done much easier not using Selection.

Sub ReplaceAndCount()
Dim r As Range
Dim counter As Long
Set r = ActiveDocument.Range
With r.Find
   .ClearFormatting
   Do While .Execute(Findtext:=",11,", Forward:=True) = True
       With r
           .Text = "11"
           .Collapse Direction:=wdCollapseEnd
       End With
       counter = counter + 1
   Loop
End With
Set r = Nothing
MsgBox counter & " replacements made."
End Sub

And of course it could be used to take any two input strings as parameters.

Sub ReplaceAndCount(strOld As String, strNew As String)
Dim r As Range
Dim counter As Long
Set r = ActiveDocument.Range
With r.Find
   .ClearFormatting
   Do While .Execute(Findtext:=strOld, Forward:=True) = True
       With r
           .Text = strNew
           .Collapse Direction:=wdCollapseEnd
       End With
       counter = counter + 1
   Loop
End With
Set r = Nothing
MsgBox counter & " replacements made."
End Sub

Sub GiveStrings()
   Call ReplaceAndCount(",11,", "11")
End Sub

With all due respect to the Word MVP org bunch, counting ALL the characters
in a document to get a replacement counter seems utter madness to me.  And
completely not needed.

Further, even the word count function seems way over the top.  It needs to
turns screen updating off (because it uses Selection).  It counts all the
characters....TWICE!  It makes REAL replacement of text....then has to do an
Undo to put the text back.  Then put screen updating back on.  All for a word
count????  What on earth does doing a text replacement have to do with
counting words?

Counting all the characters?  Ummm, in a 500 page document...gee, that sounds
very efficient.

Function MyWordCount(strIn As String) As Long
Dim r As Range
Set r = ActiveDocument.Range
With r.Find
   .ClearFormatting
   Do While .Execute(Findtext:=strIn, Forward:=True) = True
       MyWordCount = MyWordCount + 1
   Loop
End With
End Function

Sub GetWordCount()
Dim strIn As String
strIn = InputBox("Word, or phrase?", "My text counter")
   MsgBox "There are " & MyWordCount(strIn) & _
           " instance of the word, or phrase:   " & _
           strIn
End Sub

No screen updating changes required, as there is no use of Selection.  No
replacement of text required.  No Undo of those replacements required.  No
calculations required...simply....a counter.

Found one.....counter + 1
Found one.....counter + 1

Display the count.

Done.