You cannot use MAX() in a WHERE clause, instead use a subquery. The ORDER BY is also unneeded, because we only select the maximum. So what to order, there is only one number (even if multiple entries with the same number).
@@ -1,5 +1,4 @@
SELECT berater_id, name, anzahl
FROM Beratungsanzahl
JOIN Berater ON Berater.berater_id = Beratungsanzahl.berater_id
- WHERE anzahl = MAX(anzahl)
- ORDER BY anzahl DESC
+ WHERE anzahl = (SELECT MAX(anzahl) FROM Beratungsanzahl)