As to the large number of Welsh saints D.H. Farmer, in The Oxford Dictionary of Saints, says that  "the Celtic countries had shown persistent tenacity to local tradition and had been less susceptible to Roman influence than England ... In their cult of saints, as in some other matters, Celtic traditions had developed somewhat differently than elsewhere in the West. ... In these countries the word (saint) had come to mean hardly more than pious church founder or learned ecclesiastic." 

By the 11th century, when there was a demand for written documentation, many ancient records had been long destroyed in countless Viking raids. The papacy was not able to discover sufficient detail of the lives and acts of the early Welsh saints and in many cases their only memorials are place names and 19th century "restorations" of tiny churches on land held sacred to their names by local communities for a thousand years or more. 

St. David is the only Welsh saint to be canonized and culted in the Western Church. He has been the patron saint of Wales since the 12th century, but very little is known about his life. He died in 589 or 601 after founding a monastery in the area of Pembrokeshire which now bears his name, and living an austere life devoted to God. He is first to be found in an Irish Catalogue of Saints dating from around 730 and by 800 his feast day was determined as March 1st. 

By the 9th century he had gained the name Aquaticus because he and the monks of his establishments were supposed to have drunk only water. His earliest Life* appeared around 1090 and was composed by a son of Sulien, bishop of St. David's. The aim of this work was to promote the independence of the Welsh church. The Life tells us that St. David founded ten monasteries (including Glastonbury) and that the monks were vegetarian. Their regime included manual labour, study and worship. 

There are more than fifty Welsh churches dedicated to St. David and these are all in south Wales. The greatest concentration is in the south west and D. Simon Evans makes the point that his cult seems to have spread eastwards along what remained of the Roman road system of Wales. 
 

*Rhygyvarch's  Life of St. David.

The Welsh Life of St. David, edited by D. Simon Evans, deals with 14th century Welsh texts based on earlier Latin versions of Rhygyvarch's Life. (U. of Wales Press, 1988, ISBN 0-7083-0995-X.)