Do Vedas describe about the 11 rudras?
What are the names of those rudras according to Vedas?
How did they emerge from the supreme Rudra?
No, they are not 11 Rudras mentioned in the Vedas. This is a concept invented in the Puranas. Specifically, Matsya(५.२९-३॰), (Harivamśa (१.३.४९-५२) and has no support in the Vedas.
Rig Veda reveals Rudra as श्वितीचे (śvitīce) (२.३३.८) which is translated by Sāyaṇa as of white complexion.
He is one among two or three gods who are invoked as having or assuming all forms. Another epithet for Rudra is varāha (१.११४.५) दिवो वराहमरुषं कपर्दिनं त्वेषं रूपं नमसा नि ह्वयामहे which is explained by Sāyaṇa one who has good food.
Rudra bestows happiness on the sons and grandsons of the invoker (२.३३.१४) परि णो हेती रुद्रस्य वृज्याः परि त्वेषस्य दुर्मतिर्मही गात् ।अव स्थिरा मघवद्भ्यस्तनुष्व मीढ्वस्तोकाय तनयाय मृळ and grants a longer life and progeny (२.३३.१-२) अभि नो वीरो अर्वति क्षमेत प्र जायेमहि रुद्र प्रजाभिः
Another important aspect of Rudra is his power to heal.
Prayers are presented to Rudra to provide medicines (२.३३.१२) कुमारश्चित्पितरं वन्दमानं प्रति नानाम रुद्रोपयन्तम् । भूरेर्दातारं सत्पतिं गृणीषे स्तुतस्त्वं भेषजा रास्यस्मे and he is the Chief among all the physicians (२.३३.४ and १.११४.१). मा त्वा रुद्र चुक्रुधामा नमोभिर्मा दुष्टुती वृषभ मा सहूती । उन्नो वीराँ अर्पय भेषजेभिर्भिषक्तमं त्वा भिषजां शृणोमि ॥४॥
Rudra is described as the Lord of the world (२.३३.३-४) श्रेष्ठो जातस्य रुद्र श्रियासि and nobody is possessing more power than him (२.३३.१॰). अर्हन्निदं दयसे विश्वमभ्वं न वा ओजीयो रुद्र त्वदस्ति
In Rig Veda Rudras were mentioned as Maruts.
Rig Veda I.39 was dedicated to Maruts.
नहि वः शत्रुर विविदे अधि दयवि न भूम्यां रिशादसः |
युष्माकम अस्तु तविषी तना युजा रुद्रासो नू चिद आध्र्षे || Rig Veda I.39.4
Consumers of your foes, no enemy of yours is found in heaven or on the earth: Ye Rudras, may the strength, held in this bond, be yours, to bid defiance even now.
And, Maruts were mentioned as the children of Rudra and Pṛśni (Rig veda VI.66.3)
They who are Sons of the rain-pouring Rudra, whom the long-lasting One had power to foster: The Mighty Ones whose germ great Mother Pṛśni is known to have received for man's advantage.
According the Wikipedia article, the number of Marutas varies from 27 to sixty .
They are very violent and aggressive, described as armed with golden weapons i.e. lightning and thunderbolts, as having iron teeth and roaring like lions, as residing in the north, as riding in golden chariots drawn by ruddy horses.