OS. Use Geometry Methods

After the mStaadObj declaration, type Dim nodesCount As Integer and then press Return.

This dimensions the variable nodesCount as an integer.
Type Dim beamCount As Integer and then press Return.

This dimensions the variable beamCount as an integer.
After the mStaadObj initiation, type nodesCount = mStaadObj.geometry.GetNodeCount() and then press Return.

This uses the OpenSTAAD method GetNodeCount() to return the number of nodes in the active STAAD.Pro model into the specified variable.
Type beamCount = mStaadObj.geometry.GetMemberCount() and then press Return.

Similarly, this uses the OpenSTAAD method GetMemberCount() to return the number of members in the active STAAD.Pro model into the specified variable.

Tip: Reference the OpenSTAAD API Documentation for details on available methods and what values are accepted as input or returned from each.

Your program at this point should look like:

Public Class Form1
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim mStaadObj As Object
        Dim nodesCount As Integer
        Dim beamCount As Integer

        mStaadObj = GetObject(, "StaadPro.OpenSTAAD")
        nodesCount = mStaadObj.geometry.GetNodeCount()
        beamCount = mStaadObj.geometry.GetMemberCount()
        
    End Sub
End Class