The RecordCount in a Recordset is not accurate until you do a MoveLast.
However; instead of doing that it would be more efficient to use a SQL statement
that does the count...

sql_volume = "SELECT Count(*) " & _
                   "FROM meetoefeningen " & _
                   "WHERE Meetgroep=1"
Set TB_volume = db.OpenRecordset(sql_volume)
volume = TB_volume.Fields(0)

The RecordCount is the number of records accessed so far.
When first loaded, it will be 0 (if there are no records) or 1 (if there are
any.)
To get the full count, MoveLast.

The Move methods error if there is no record to move to, so:
   Set TB_volume = ...
   If TB_Volume.RecordCount > 0 Then
       TB_Volume.MoveLast
   End If
   Debug.Print TB_Volume.RecordCount

For more info, see:
   Traps Working with Recordsets - 10 common mistakes
at:
   http://allenbrowne.com/ser-29.html