Bhagwan Sri MahaVishnu ,in one of His manifestations, is Aditya ,born of Sage Kasyapa.He declares in Srimad Bhagavad Gita ‘ Adityaanam aham Vishnu ‘,I am Vishnu among the Adityas. Adityas are twelve. Surya, the Sun, is the Eldest. In this sense , Surya,Sun, is the Jyeshta,Elder, to Vishnu.It may be noted that Suryanarayana is called the Visible Reality,Prataksha Brahma.
Therefore many things associated with Vishnu are twelve in Number.For instance, His Moola Mantra is Om Namo Bhagavate Vaasudevaaya, Dwadasanama, Twelve letters,Twelve syllables.
Similarly, Siva,in the Form of Sri Rudra, is one among the Eleven Rudras. He is called Sankara. ‘ Rudraanam Sankarosmi’- Sri Krishna in Srimad Bhagavad Gita.’Siva Dwadasa Nama Smaranam (remembering the twelve names of Lord Shiva) Translated by P.R.Ramachander (This rare collection of 12 names of Lord Shiva, was sent to me by a reader of http://www.celextel.org . He has taken it from a manuscript written in Telugu. My thanks to him.) Prathamastu Mahadevo DwitEyastu Mahesvaraha Truteeyaha Sankaro gneya Chaturtho Vrushabhadhwajaha Panchamaha KruththiVaasaascha Shashtaha Kamaanga nasanaha Saptamo Deva Devasaha Srikanasch ashtama smruthaha Eswaro navamo gneyo Dasamaha ParvatI patihi Rudra Ekadasaschaiva Dwadasaha Siva Uchyathe First the great Lord, Second the greatest among gods, Third the well known Shankara Fourth, the one who has bull in his flag, Fifth, the one who wears animal hide Sixth, the one who killed the God of love Seventh, the God of devas, Eighth the one with poison neck Ninth the one who is known as Easwara, Tenth the consort of Parvathi, Eleventh, the fearsome Rudra, And twelfth Lord Shiva dwadasaithani naamaani trisandhyam yah pathennaraha Kruthghnaschaiva Goghnascha bramhaga Guruthalpagaha StrI bala ghatakaschiava Suraapo VrushaLi patihi Muchyate Sarva Papebhyo Rudra Lokam Sa gaChathi. The man who reads these twelve names during dawn, noon and dusk, Even if ungrateful or having killed a cow, Even if he has killed Brahmins or teacher, Or it is a lady, child or an achiever, The lord of devas, The lord of the bull, Would get rid of all his sins, And at the end reach the land of Shiva,
The above transaction is by Sri P.R.Ramchander, an erudite scholar, who has translated nearly all Sanskrit Slokas into English. My Namaskaarams to him.