The number and variety of charms you throw in your barmbrack is a personal decision, but you don’t want so many, eaters are stumbling over a charm every other bite. Of all the charms I purchased, I only used twelve in my first barmbrack. You could make more than one barmbrack. Or another (heretical) idea is to make muffins, instead of one large barmbrack, and stick one, two, or three charms in each muffin before you bake it. People’s fortunes for the coming year would then be a synthesis of whatever charms they found in their muffin.
Just know what each charm means when you select it for purchase. A posted list of meanings somewhere nearby will help people interpret their charms. If more than one charm turns up in their serving, it could be referring to more than one event, or the charms could be combining to deliver the same message. For example, below is a picture of the charms I found in my first slice of barmbrack:
I interpreted this as a message that someone deceased is sending me love. Not a prediction of my coming year, but a warm and welcome message all the same.
Gemma of Bigger Bolder Baking says you’re supposed to wrap each charm in a small square of baking parchment. This preserves the mystery, prevents any chemicals in the charm from leaching into the bread, and alerts people to the presence of a charm. Tell people to be on the look-out for the parchment bundles in their serving.
I’ve seen pictures of barmbrack baked in a regular loaf-pan, but it seems barmbrack was traditionally baked in a round pan, then cut into very long, thin slices when cooled. You will probably want to factor in both the shape of your pan, and the manner in which you’ll be cutting it, into your charm-distribution. As always, say a prayer to Whomever you usually do before you start preparing the batter and inserting the charms, asking Them to guide your hands as you do this.