This seems to be the day for self join relationships. This irequest is very similar to: How can I count the values but there the poster wants to count records from the same month.
I'm going to call your table "Children".
- Open manage | Database | Relationships.
- Click the box labeled "Children" to select it
- Click the button on bottom left with two green plus signs on it to make second "table occurrence" of this table. You haven't created a new table, just a "box" on this window that also refers to Children.
- Drag from the GroupChar field in one table occurrence box to the GroupChar field in the other. This will give you this relationship:
Children::GroupChar = Children 2::GroupChar
- Now click the Fields tab and create a calculation field with this calculation: Count ( Children 2::GroupChar)
- Place this calculation on your layout and you'll see the count for all children records with the same group character as your current record.
You can also click the Fields tab in Manage | Database and double click GroupChar where you can use Count ( Children 2::GroupChar) in a validation calculation that will pop up an error when the count exceeds 10:
Count ( Children 2::GroupChar) < 11
If you'd like to learn more about table occurrences and how they control the function of FileMaker databases: