Rig veda VII.91.7 says :
अर्व॑न्तो॒ न श्रव॑सो॒ भिक्ष॑माणा इन्द्रवा॒यू सु॑ष्टु॒तिभि॒र्वसि॑ष्ठाः । वा॒ज॒यन्त॒: स्वव॑से हुवेम यू॒यं पा॑त स्व॒स्तिभि॒: सदा॑ नः ॥७॥
We, VASISHTHAS, (bearing oblations) like horses (bearing burthens), soliciting food, desiring strength, invoke with praises INDRA and VAYU for (our) sure defence: do you ever cherish us with blessings.
The rishi is Vasistha.
My question is why the poet stated in the mantra Vasishthas (plural form), instead of singular form?