The Rambam establishes the foundational rule that a scribe must be precise about tagim and their count, noting that some letters carry one tag while others carry seven, and that all tagim are formed in the shape of zayin letters, thin as a hairsbreadth — leaving the specific count on any given letter to be transmitted scribe-to-scribe (Mishneh Torah, Hilchot Tefillin 7:8).
The tradition of tagim is associated specifically with the letters שעטנ"ז ג"ץ, as רש"ר הירש, דברים ו ט notes that crowns appear on precisely those letters, reflecting a teaching found in the Gemara about those letters being adorned with crown-like ornaments.
Regarding the physical form of the tagim, Rav Mordechai Eliyahu (Maamar Mordechai Hilchot Stam 10:18) cites the Mishnah Berurah to the effect that each tag should ideally be shaped like a zayin, projecting on both sides, though some are accustomed to making each one merely a thin line; and he further records that the three tagim on the letters שעטנ"ז ג"ץ consist of one to the right, one to the left, and one on top — with an alternate opinion holding that all three are placed on top, which is the accepted custom.
Rav Ovadia Yosef (Maayan Omer 6:3:35) addresses the specific question of the beis of Bereishit directly, reporting that the custom in Baghdad was to place two tagim on that letter, as cited by the Beis Yosef in the name of Baruch, while expressing uncertainty about whether this reflects two distinct customs or a single established practice.
