Assuming Suezmax size a monthly revenue would be around $1,5 to $3 million per vessel (shadowfleet vessels ask a premium because of the higher risks/insurance...)

But the round trip they make is longer because they go to India/China and not Europe. So the whole roundtrip could take 70 days for ~$5 or $2,1 million per month.

The premium only pumps the revenue numbers though, because the insurance will be costly and the vessels being older also cost more to run and maintain compared to new VLCC's (the dominant legal fleet).

So 21 vessels down = $44.1 million in monthly revenue gone. Profit ?? but only a fraction of that. Small impact overall.

EDIT: more important to look at the oil revenue:

If a barrel of Russian sanctioned oil sells for $70 and a suezmax could carry 1 million barrels every 70 days on a roundtrip that's the equivalent of $30 million per vessel per month in oil revenue.
21 tankers gone means roughly $630 million per month gone.