Given that you're only converting to ints to then perform a comparison, I'd just switch the table definition around to using varchar also:
Create table #myTempTable
(
num varchar(12)
)
insert into #myTempTable (num) values (1),(2),(3),(4),(5)
and remove all of the attempted CONVERTs from the rest of the query.
SELECT a.name, a.value AS value, COUNT(*) AS pocet
FROM
(SELECT item.name, value.value
FROM mdl_feedback AS feedback
INNER JOIN mdl_feedback_item AS item
ON feedback.id = item.feedback
INNER JOIN mdl_feedback_value AS value
ON item.id = value.item
WHERE item.typ = 'multichoicerated' AND item.feedback IN (43)
) AS a
INNER JOIN #myTempTable
on a.value = #myTempTable.num
GROUP BY a.name, a.value ORDER BY a.name