You will need a macro in the ThisDocument module of the document, and save the document as macro enabled. Name the dropdown 'Room' and the text CC as 'Seats' then change the case statements to reflect the number of seats for each room in the list (only three shown in the example). Then when you leave the dropdown, the text field is updated according to the selection.
Code:
Option Explicit
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim oCC As ContentControl
For Each oCC In ActiveDocument.ContentControls
If oCC.Title = "Room" Then
Select Case oCC.Range.Text
Case "Room 1"
ActiveDocument.SelectContentControlsByTitle("Seats").Item(1).Range.Text = "63 Seats"
Case "Room 2"
ActiveDocument.SelectContentControlsByTitle("Seats").Item(1).Range.Text = "25 Seats"
Case "Room 3"
ActiveDocument.SelectContentControlsByTitle("Seats").Item(1).Range.Text = "42 Seats"
Case Else
ActiveDocument.SelectContentControlsByTitle("Seats").Item(1).Range.Text = ActiveDocument.SelectContentControlsByTitle("Seats").Item(1).PlaceholderText
End Select
End If
Next oCC
lbl_Exit:
Set oCC = Nothing
Exit Sub
End Sub