Rig-Veda 1.64 is riddle Hymn. Also called as "asya vamsaya" hymn . The vedic rishi Dirghatamas is the seer of the hymn and is considered by scholars much difficult to interpret as it presents series of riddles before readers.
H.H. wilson whose translation is based on Sayanacharya Bhashya is supplying us the following explanation about the sukta and mantra 2-4. Mostly the commentary is self explanatory.
Here is Sayana's general view about sukta.
According to Sayana , however , the general purport of this sukta is
the inculcation of the doctrines of the "Vedanta" or the spiritual
unity of Brahma and the universe : some passages occur that bear him
out in this view , but the text , upon the whole ,although often
mystical and obscure ,evidently proposes the glorification of "Aditya"
, or the sun , especially as identifiable with all creation.
Now let's see the translation of H.H, Wilson and commentary by sayana of each mantra.
सप्त युञ्जन्ति रथमेकचक्रमेको अश्वो वहति सप्तनामा |
चक्रमजरमनर्वं यत्रेमा विश्वा भुवनाधि तस्थुः || Rig-Veda 164.2 ||
sapta yuñjanti rathamekacakrameko aśvo vahati saptanāmā |
trinābhi cakramajaramanarvaṃ yatremā viśvā bhuvanādhitasthuḥ ||
Translation - They yoke the seven (horses) to the one wheeled car : one horse , named seven , bears it along ; the three -axled wheel is
undecaying , never loosened , and in it all these regions of the
Sayana interpretation -
रथमेकचक्र - "One wheeled car" - Either the orb of the sun , or time ,
or a year : the seven horses may be the seven solar rays , or the six
seasons , with their aggregation and year.; or the six double months ,
and inter-calary month ; or the seven days of the week : the wheels of
the car , as typical of time , past , present , and future , should be
three , but they are identical in nature , and are therfore said to be
चक्रमेको अश्वो वहति सप्तनामा - "to the one wheeled car : one horse ,
named seven " - Eko aswo saptamaha may mean the sun , or Aditya
either as the absorber of seven flavors , or as praised by the seven
Rishis ; or it may be a sort of pun , sapta meaning a horse as well as
त्रिनाभि -trinābhi - The day with three sandhya's ; the year with
three seasons , hot , wet and cold.
विश्वा भुवनाधितस्थुः viśvā bhuvanādhitasthuḥ - All things are
dependant on time ; which itself is imperishable , as the smiriti ;
anadinidhanah kalah , time is without beginning.
इमं रथमधि ये सप्त तस्थुः सप्तचक्रं सप्त वहन्त्यश्वाः |
स्वसारो अभि सं नवन्ते यत्र गवां निहिता सप्त नाम || Rig-Veda 164.3
imaṃ rathamadhi ye sapta tasthuḥ saptacakraṃ sapta vahantyaśvāḥ |
sapta svasāro abhi saṃ navante yatra ghavāṃ nihitā sapta nāma ||
Translation - :The seven who preside over this seven-wheeled chariot
(are) the seven horses who draw it ; seven sisters ride in it
together , and in it are deposited the seven forms of utterance.
Sayana's interpretation -
सप्त - sapta - Either the solar rays , or if the year be typified ,
the seven portion of it ; as the ayana or solstice , season , month ,
fortnight ,day , night hour. Sayana considers the seven wheels as
well as the seven horse to be the solar rays.
Seven sisters - : The commentator repeats the interpretation either he
rayas of the sun , or the six seasons and the aggregate year , or the
six double and one intercalary month.
गवां ghavāṃ - : seven forms of utterance - The seven notes of music
as employed in chanting the praises of the sun ; or if " gavam" be
used in sense of water ,the seven forms may be seven divine rivers.
को ददर्श प्रथमं जायमानमस्थन्वन्तं यदनस्था बिभर्ति |
असुरस्रुगात्मा क्व स्विक्तो विद्वांसमुप गात्प्रष्टुमेतत || Rig-Veda
ko dadarśa prathamaṃ jāyamānamasthanvantaṃ yadanasthā bibharti |
bhūmyā asurasṛghātmā kva svit ko vidvāṃsamupa ghāt praṣṭumetat ||
Translation - Who has seen the primeval (being) at the time of his being born ; what is that endowed with substance which the
unsubstantial sustains ; from earth are the breath and blood , but
where is the soul; who may repair to the sage to ask this.
Sayana's interpretation -
Asthanwantam yad anastha vibhartti - lit, that which having bone , the
boneless sustains ; the latter , according to sayana is the prakriti
of tha sankhyas , or the maya of the vendantis ,formless matter , or
spiritual illusion , from which the material and visible world
The mantras 2-4 are dedicated to aditya or the sun . The sun is also symbol of supreme light or supreme sun god. The sun or supreme light has seven children , the visible sun , Moon and five planets.
Here is Rig-Veda Subodha Bashya in hindi by Shripad Damodar Satwalekar translated by Narinder Sharma.
So if we look above we can find the complexity of the sukta. A single word used in same mantra multiple times , can have multiple meanings.i.e. The use of the number seven recurs throughout the hymn. As you have enquired about the word Seven or Sapta. The word can be interpreted variousely throughout the hymn .Like The seven notes of music ,seven days of the week , seven solar rays , seven sisters , seven rivers , seven rishi's etc. But Who the seven is not precisely known.
Conclusion - As you have stated , according to sayana this hymn is resembling the teaching of upanishads i.e. vedanta that Brahman and
the universe is same. According to sayana the Prakriti of Samkhya or
Maya of vedantis (advaita) can be traced in mantra 4 ,(that which
having bone , the boneless sustains ) . The seven word can be
visualised as seven rays residing in sun are seven horses ,which took
the sun everywhere. The seven name are the seven swaras . seven
sisters are the seven Chandas (छन्द) with which vedas praise the sun