Compounds ending in 1 2 and 3 are the only ones to decline: I saw 20 blackbirds = vīgintī merulās vīdī I saw 22 blackbirds = vīgintī duās merulās vīdī (where duās changes to agree with merulās) .
However, 1000 is mille, an indeclinable adjective, but multiples such as duo mīlia or duo mīllia (2000) have mīlia as a neuter plural substantive followed by a partitive genitive: I saw a thousand lions = mīlle leōnēs vīdī I saw three thousand lions = tria mīlia leōnum vīdī .