Im not sure there is a formula but vba could do just what you need.
Sub countBoldCells()
Const whatColumn = "A"
Const whatSheet = "Sheet1"
Dim xlWS As Worksheet
Dim lastRow As Long
Dim boldCounter As Long
Set xlWS = Worksheets(whatSheet)
lastRow = xlWS.Range(whatColumn & Rows.Count).End(xlUp).Row
boldCounter = 0
For r = 1 To lastRow
If xlWS.Range(whatColumn & r).Font.Bold = True Then
boldCounter = boldCounter + 1
End If
Next
MsgBox "There are " & boldCounter & " bold cells."
End Sub