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