The Mabinogi are structured into four, the Four Branches, Pedair Cainc.
Welsh literary tradition also lays great emphasis on the number three, as in the Trioedd, the Triads. These were triple sets of people, or places animals or objects, linked by a common quality e.g. the Three High Matriarchs of Britain. Some of these are referred to in the Mabinogi.
Numbers have a long history going back to the most ancient magical systems of symbolising much more than arithmetic. In numerology a number corresponds to a cosmic entity, usually making the Sun number 1, symbolising central things like leaders, the family hearth. Numbers also correspond to people and events in well known texts like the Bible, or major poetry. So to a given audience, a number gains a rich background of meanings.
There are various significant numberings in the Mabinogi and this is a list, though it cannot be comprehensive for more will keep turning up!
All Four Branches: doublets, two parallels of plot structure, were identified by John Bollard. These pairs of 2 reflect a similar or polarised situation in different tales, and can suggest deeper meanings.
Triads, Trioedd: uniquely Welsh poetic sayings about 3 items. Some are used as Mabinogi references e.g. Manawydan one of the Three Humble Chieftains.
The Mabinogi is divided into 4, Four Branches, Pedair Cainc. Each contains several tales and they are all quite closely interconnected, or ‘interlaced.’
There are 4 cantrefs of Ceredigion.
The Cauldron in the Second Branch is broken in 4 pieces (cf. the tale of Gwion)
Second Branch: 5 pregnant women were all that survived the Irish War on the Irish side.
Rhiannon has 6 treacherous maids who frame her with puppy blood as an infanticide to avoid blame.
First branch: there 7 cantrefs of Dyfed, and 7 cantrefs of Seisyllwch. Rhiannon’s penance is 7 years.
Second Branch: 7 Riders are left to rule Britain during the Irish War. There are 7, Seven Survivors of that war.
The collection called “The Mabinogion” is typically 11 stories from mediaeval manuscripts of 13th -15thC Wales. Four of these group together as the Mabinogi, though each of the Four Branches contain several episodes.
Second Branch: the Irish War had Seven Survivors on the British side, and five pregnant women survived on the Irish side. Together that is 12, with Branwen 13; but Branwen died of grief on returning home to Harlech.
Second Branch: 13 ships arrive at Harlech from southern Ireland to open events of the Second Branch. Branwen makes 13 survivors of the Irish War, see under 12.
First Branch, Fourth Branch: a cantref is a portion of territory based on 100, probably 100 fighting men as a levy.
Pwyll commands 100 fighting men at Rhiannon’s wedding feast, although the way the Mabinogi counts a large group like this means it was 99 men plus Pwyll himself.