How to prevent users insert the same data when the quantity of the data has reached its limit?

Question

I'm using VB.net and SQL for my project. I give a situation for my problem:

Table Quantity

id    Pro_no    Quantity 
1      123          4
2      456          5

Table insertData

id    Pro_no     Quantity    Description    
1      123           2         broken
2      123           1       missing part

As you can see user has inserted Pro_no for 123 into the insertData table. Meaning that it has 1 more Quantity can be inserted. My problem is when there are 2 users OR user open the same data with 2 tabs, want to insert the data at the same time, one of them OR one of the tab will not successful to do it cause it has reached the limit.

SQL :

INSERT INTO insertData (Pro_no, Quantity,Description)
VALUES ('123','1','broken')

VB.net

    Protected Sub btnCreate_Click(sender As Object, e As EventArgs) Handles btnCreate.Click
        If ErrorChecking() = True Then
            DataInsert()
            DisplayMessage("success", "Done", "Data Succesfully Inserted!")
            clearAll()
        End If
    End Sub

DataInsert() Function :

Private Function DataInsert() As Integer
        Using create As New clsQuantityProject_dal
            Dim obj As New clsQuantityProject_info
            With obj
                .Part_No = txtPartNo.Text
                .Description = txtDesc.Text
                .QTY = spinEditQTY.Text
            End With

            Dim strResult As Integer = create.QuantityDataInsert(obj)

        End Using
        Return 1
End Function

note: do tell me if you can't understand my question, I'll correct and explain it better