What is an epic and why is there “only one epic in English Language so far”?

There are many epics in English besides Paradise Lost. Technically speaking, the "epic" is a narrative mode rather than a genre. What makes a narrative "epic" is a distinction between subject and object: the speaker (the subject doing the telling) is not the topic (the object being treated). Compare with the lyric, in which poets write about themselves (i.e. the subject is also the object) or the drama, in which there is no narrative voice (i.e. there is an object being treated but not a subject doing the telling). What makes narrative poems such as the Iliad and the Odyssey epic is simply the fact that there is a narrator relating events external to his or her own subjective experiences.

But these poems also had additional features that became commonly associated with "the epic," and we find Aristotle, in the Poetics, briefly enumerating them:

Epic poetry agrees with Tragedy in so far as it is an imitation in verse of characters of a higher type. They differ, in that Epic poetry admits but one kind of metre, and is narrative in form. They differ, again, in their length: for Tragedy endeavours, as far as possible, to confine itself to a single revolution of the sun, or but slightly to exceed this limit; whereas the Epic action has no limits of time.

Epics, in addition to being "narrative" (i.e. in addition to including a storyteller), were also long compositions in verse about "higher characters." Thanks in part to Aristotle's influence, it became conventional in the Renaissance to use the term "epic" to also designate a genre, a type of narrative characterized by a certain length, style, and subject matter. Epics tended to be long poems on grand subjects (the crusades, in Tasso's Jerusalem Delivered, the age of navigation in Camões's The Lusiads, or Genesis, in Milton's Paradise Lost). They were commonly distinguished from the tradition of romance, which were also long verse narratives characterized by a focus on individual stories of courtly love rather than collective military or heroic deeds.

Now, because the epic is above all a mode, authors like Miguel de Cervantes, in Spain, and Henry Fielding, in England, made the case that epics could be written in prose as well as verse -- and, implicitly, that works like Cervantes's Don Quixote and Fielding's Joseph Andrews were epics. From this standpoint, all novels are epic. The usual claim that a novel like Gone with the Wind is an epic harks back to a Renaissance argument that epics could be written in prose.

But even if we look at the epic more strictly as being a long heroic poem, there are many epics in English literary history. They just don't happen to be as highly canonical as Paradise Lost. Less canonical, but still quite canonical, are Beowulf, James Macpherson's forgery of the Ossian poems, and Keats's Hyperion; less so are the variety of mock-epics in eighteenth-century satirical verse or serious (but second-rate) epics such as William Wilkie's Epigoniad, which you can check out on Google Books.

Since Milton is often discussed in the context of Renaissance literature, I'll quote the definition of "epic" from The Renaissance (edited by Marion Wynne-Davies, Bloomsbury Guides to English Literature, Bloomsbury, 1992):

A narrative of heroic actions, often with a principal hero, usually mythical in its content, offering inspiration and ennoblement within a particular cultural or national tradition.

John Milton wrote two works that fit this definition, namely Paradise Lost and Paradise Regained, thereby disproving the claim that he wrote "the only epic composed in English Language". Another Renaissance poet, Edmund Spenser, wrote the epic poem The Faerie Queene (published in the 1590s).

In addition, Wikipedia lists more than 40 epic poems in the English language, including the following:

• Geoffrey Chaucer's Troilus and Criseyde (1380s),
• William Blake's Milton: A Poem in Two Books (1804-1810),
• John Keats's The Fall of Hyperion: A Dream (1850s),
• Henry Wadsworth Longfellow's The Song of Hiawatha (1855),
• Algernon Charles Swinburne's Tristram of Lyonesse (1882),
• William Carlos Williams's Paterson (1940s-1950s),
• Derek Walcott's Omeros (1992).

If a handbook claims that John Milton wrote the only epic poem in the English language, its author or authors are wrong. They may not be competent enough to write about the history of English literature.