Related for Berakhot 7:4

שְׁלשָׁה שֶׁאָכְלוּ כְאֶחָד, אֵינָן רַשָּׁאִין לֵחָלֵק, וְכֵן אַרְבָּעָה, וְכֵן חֲמִשָּׁה. שִׁשָּׁה נֶחֱלָקִין, עַד עֲשָׂרָה. וַעֲשָׂרָה אֵינָן נֶחֱלָקִין, עַד שֶׁיִּהְיוּ עֶשְׂרִים:

If three ate together, they are not permitted to separate, [having become subject to the obligation of zimun]. And the same applies to four or five. [Three are not permitted to make the zimun and one to separate, he, too, having become subject to the obligation of zimun.] Six may separate [(three) for zimun in one group and three) in another], until ten; but ten may not separate, [their having become subject to zimun with mentioning of the Name], until there are twenty, [at which time they may separate into two groups (of ten) if they wish].

Tosefta Berakhot