Answered a few months ago by /u/alriclofgar in this here thread.

It's worth adding a couple of basic points: the main British migrations to Brittany took place in the period roughly 400-550 – while the claim that the Bretons were "all too eager to take part in the conquest of England" in order to obtain revenge against the Saxons is sometimes made, this appears to be nothing more than supposition; certainly there is no contemporary evidence that this was so; and modern historians of the Saxon "invasions" prefer to see the process as in large part a peaceful migration, involving much intermarriage, in any case. We know that 11th century Breton magnates were conscious of their heritage; in their charters, the likes of Rivallon of Dol-Combour and Haimo of Sougéal describe themselves in terms such as "Riuallonius, Britannicus gente" or "Haimo, patria Brito. But in excess of half a millennium is a very long time for any sort of "ancestral memory" – or grudge – to apply.

It's remarkably rare to find any sustained analysis of why Bretons fought for William. The fact that the Normans had fought a two year war against Brittany in 1064-66 does not seem to have resulted in any formal requirement for the Bretons who fought on either side to supply contingents to William's army; rather, William's reputation as a successful and able commander seems to have attracted men who were essentially mercenaries to his banner. It's certainly notable that of the Breton leaders identifiable among those who fought at Hastings, most came from the eastern part of the country (Snyder The Britons p.228) – the part that was not only closest to Normandy, but also furthest from the Celtic heartlands of Brittany, which lay far to the west. For all the pretensions of Rivallon and Haimo, these eastern Bretons were for the most part at least Franco-British or Normano-British in terms of their genetic heritage; many held extensive lands in Brittany, yet had little if any identifiably Celtic blood.

It may be more useful to investigate contemporary connections than ancient lineages; for instance, Edward the Confessor had a servant known as Ralph the Staller, a royal steward who became part of William the Conqueror's entourage after the conquest. Although probably born in East Anglia (Dictionary of National Biography), Ralph had a Breton father who may have accompanied Emma of Normandy when she came to England to marry Æthelred II in 1002. He owned lands on both sides of the Channel, including a single manor in Cornwall. His son, Ralph de Gael, inherited both Breton lands and an English earldom from his father, and was a leading figure in the major unrest that occurred in 1075 – note, however, that in this rebellion he allied with a surviving Saxon noble, Earl Waltheof, against William.

Another and perhaps more interesting datum is that Brian, son of the Breton landowner Eudo of Penthièvre, took part in the invasion of 1066; he was given lands in the far south-west of England as a reward and may have been named Earl of Cornwall (Douglas, William the Conqueror p.267.) Was this some sort of recognition of distant kinship with the Cornish? Might the Conqueror have hoped he would be effective in binding Cornwall to him after 1066? We simply have no evidence either way. But Brian's elder brother Alan the Red, who also fought at Hastings, and was more richly rewarded for it than was Brian, received his lands in Yorkshire, Lincolnshire and East Anglia – so we certainly can't say that settling Bretons in predominantly Celtic areas of England was William's invariable policy, or that William saw Cornish lands as choice rewards for his most important Breton supporters.

As to total numbers: we have no exact numbers for the make up of William's army, either, but it is generally put at around 8,000 men, and it consisted of a mixed group of Normans backed by Bretons, Flemings and French mercenaries; even though the Bretons are generally supposed to have supplied the largest of these non-Norman contingents, a thousand or two thousand men from Brittany would seem a reasonable maximum. Also worth noting: their influence in post-conquest England was not substantial. Keats-Rohan, in the only study explicitly devoted to Bretons in England in the post-conquest period ("The Bretons and Normans of England 1066-1154: the family, the fief and the feudal monarchy," Nottingham Medieval Studies, 1992), observes that "the non-Norman element has generally been regarded as too small to warrant more than isolated comment," and adds that "between 1066 and 1075 there were three groups of Breton landholders in England. After 1075 there were only two."