Customizer → Graphics → Link groups.

A link group is a single text input ("Number") whose value is rendered at every member position at once, each with its own font size and color override. Typical: number on front (small) and back (large); name on back and left sleeve.
Link groups are assigned to products (and optionally designs) and can carry a roster role so team rosters fill them automatically.
One position, one group
A position can belong to only one link group. If a position is missing from the picker it is already used elsewhere or has the wrong content type.
The front and back numbers are different sizes — is that a bug? No, that is the per-member font size override. Clear it to use the same size everywhere.
Can the customer position the number? No. Linked text is fixed at its positions. For movable text, use a text zone or freeform text.
Name and number on the same jersey. Create two link groups (one text with role Player name, one number with role Player number) and assign both.