If you need the number of days for each month, you can use an array that is inialised with zeroes to store the numbers for each month. You can then reference the values for each month (by index 0 to 11). You will also need to change the query to return the month number.
SqlCeCommand commandArbeitstage= new SqlCeCommand("select datepart(month, Datum) as Month, count(IDStundensatz) as Gesamt from tblstunden Where IDPersonal = @IDPersonal Group by datepart(month, Datum) Order By datepart(month, Datum)", verbindung);
commandArbeitstage.Parameters.Add("IDPersonal", SqlDbType.Int);
commandArbeitstage.Parameters["@IDPersonal"].Value = IDPersonal;
SqlCeDataReader readerArbeitstage = commandArbeitstage.ExecuteReader();
Int32[] Arbeitstage = new Int32[12];
while (readerArbeitstage.Read())
{
Arbeitstage[readerArbeitstage.GetInt32(0) - 1] = readerArbeitstage.GetInt32(1));
}
textBox53.Text = Arbeitstage[0].ToString(); // January
// ... and so on up to 11