Maybe there are easier ways but here are a couple ideas
This one counts Non-Blank Cells of all Sheets
----------------------------------------------------------
Private Sub Workbook_Open()
CountCells
End Sub
Public Sub CountCells()
Dim x As Integer
Dim c
Dim WS
x = 0
For Each WS In ActiveWorkbook.Sheets
'Adjust the range to suit your needs
For Each c In WS.Range("A1:A24"

If c.Value <> "" Then
x = x + 1
End If
Next c
Next WS
MsgBox x
End Sub
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If you want to have it count cells from specific sheets you can modify the above code to
a) Use a Like condition. This would work if you are using
a naming convention for the sheets.
Example: Data_1, Data_2, Data_3 etc
Public Sub CountCells()
Dim x As Integer
Dim c
Dim WS
x = 0
For Each WS In ActiveWorkbook.Sheets
'Adjust the Name Convention to suit your needs
If WS.Name Like "Data_*" Then
'Adjust the range to suit your needs
For Each c In WS.Range("A1:A24"

If c.Value <> "" Then
x = x + 1
End If
Next c
End If
Next WS
MsgBox x
End Sub
OR
b) Use an Array. Here you specify which sheets
Public Sub CountCells()
Dim WS
Dim c
Dim i As Integer
Dim x As Integer
'Change Array contents to suit your needs
WS = Array("Sheet1", "Sheet2", "Sheet3"

For i = 0 To UBound(WS)
'Adjust the range to suit your needs
For Each c In Sheets(WS(i)).Range("A1:A24"

If c.Value <> "" Then
x = x + 1
End If
Next c
Next i
MsgBox x
End Sub
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If you need to SUM the actual contents of Non-Blank cells then change the code from: x = x + 1 to read
x = x + c.value
Also if this is the case then you might want to add the line On Error Resume Next in case any cells were to contain non numeric values.