Before the Treaty of Union in 1707, Scots was the state language of Scotland used for all government business. Even after the Union, Scots continued in use by the Scottish courts for much of the 18th century. (Wikipedia-Lallans) Scots is the Germanic language variety spoken in Lowland Scotland and parts of Ulster (where the local dialect is known as Ulster-Scots). It is sometimes called Lowland Scots to distinguish it from Scottish Gaelic, the Celtic language variety spoken in most of the western Highlands and in the Hebrides. (Wikipedia-Scots Language) Scots is at the polar opposite end of Scottish Standard English. (ibed.) Prior to the 15th century English speech in Scotland was known as “English” (written Ynglis or Inglis at the time), whereas “Scottish” (Scottis) referred to Gaelic. From 1495 the term Scottis was increasingly used to refer to the Lowland vernacular and Erse, meaning Irish, as a name for Gaelic.
The succeeding variety of Early northern Middle English spoken in southeastern Scotland, also known as Early Scots, began to diverge from that of Northumbria due to twelfth and thirteenth century immigration of Scandinavian-influenced Middle English-speakers from the North and Midlands of England. Later influences on the development of Scots were from Romance languages via ecclesiastical and legal Latin, Norman  and later Parisian French due to the Auld Alliance as well as Dutch and Middle Low German influences due to trade and immigration from the low countries. Scots also includes loan words resulting from contact with Gaelic. Early medieval legal documents include a body of Gaelic legal and administrative loans. Contemporary Gaelic loans are mainly for geographical and cultural features, such as ceilidh, loch and clan.
From the thirteenth century Early Scots spread further into Scotland via the burghs, proto-urban institutions which were first established by King David I. The growth in prestige of Early Scots in the fourteenth century, and the complementary decline of French in Scotland, made Scots the prestige language of most of eastern Scotland. By the sixteenth century Middle Scots had established orthographic and literary norms largely independent of those developing in England. As early as 1603 
a member of the Court of England wished to make a report to Mary of Guise, but when they began speaking in Scots, they had to agree to change to French as the language was unintelligible to him.
As far as how many can speak this language in Scotland, an University of Aberdeen study in 2010, while a Census program entitled “Aye Can” will take greater pains to identify this aspect of language in their 2011 Census. In Ulster (Norther Ireland), the cognate language Ullans is estimated to be spoken by 2,000 persons (again, likely undercounted). My own Great-Grandfather, 5 generations in America, still spoke a great deal of Scots, which sayings are still a part of my family’s speech. Much of American English is strongly colored by Scots, which seperated from England under our Revolution, also called the Presbyterian War (Guess the nationality referred to?). I have the Scriptures in both Scottish Gaelic and Scots (Lallans); I have ancestors who lived in both Highlands and Lowlands, therefore the interest. Even within Scots there are multiple “dialects”, the most commanding likely being “Doric”. With Scotland making increasing moves toward independence, we are likely to hear more of this language and a revival of its use and literature.