dim ObjFind as AcadObject
dim BRFind as AcadBlockReference
dim BRCount as Integer
for each ObjFind in ThisDrawing.Modelspace
if typeof ObjFind is AcadBlockReference then
Set BRFind = ObjFind
If BRFind.Name = "[Insert name of block from AutoCAD here]" Then
BRCount = BRCount + 1
End If
End If
Next
that should work, with your number of instances stored in BRCount.