You can simplify your code (and make changes to the document by running the userform again) if you adopt the following function.
As for the page numbering, you need to insert a page field in the footer and then update the start number for the current section from your text box as included below, then update the field in the footer.

Option Explicit

Private Sub NęsteButton1_Click()
Dim oFooter As HeaderFooter
Dim iPage As Integer
    FillBM "selskabsnavn1", TextBox1.Value
    FillBM "selskabsnavn2", TextBox1.Value
    FillBM "selskabsnavn3", TextBox1.Value
    FillBM "adresse", TextBox2.Value
    FillBM "postnr", TextBox3.Value
    FillBM "by", TextBox4.Value
    FillBM "cvr", TextBox5.Value
    FillBM "senesteaccept", TextBox6.Value
    FillBM "dato", TextBox7.Value
    FillBM "bestyrelse1", TextBox8.Value
    FillBM "bestyrelse2", TextBox9.Value
    FillBM "bestyrelse3", TextBox10.Value
    iPage = TextBox11.Value
    Set oFooter = Selection.Sections(1).Footers(wdHeaderFooterPrimary)
    With oFooter.PageNumbers
        .NumberStyle = wdPageNumberStyleArabic
        .RestartNumberingAtSection = True
        .StartingNumber = iPage
    End With
    oFooter.Range.Fields.Update
    Hide
End Sub

Private Sub FillBM(strbmName As String, strValue As String)
'Graham Mayor - http://www.gmayor.com
Dim orng As Range
    With ActiveDocument
        On Error GoTo lbl_Exit
        Set orng = .Bookmarks(strbmName).Range
        orng.Text = strValue
        orng.Bookmarks.Add strbmName
    End With
lbl_Exit:
    Set orng = Nothing
    Exit Sub
End Sub