Is there a way to determine if the mind conceives of only one object at a time? This is not to say that the mind only perceives of one thing at a time, but rather that it can think of only one thing at a time. Our predicate knowledge might be an indication of this. For example, when we say 'the dog is brown' there is one predicate belonging to one subject conjoined by a shifting verb 'is', where the two are understood only one at a time. And even in a predication of two things, so as to say that 'the dog is brown and red', we have a specific word to note the distinction between the two predicates, which suggests they are not thought of simultaneously or as 'one', but rather at distinct times as distinguished by the conjunction 'and'.
I read your question as being about the relation between access consciousness and subjective time.
The literal notion of now has no temporal extension, so it would appear to be trivially true that "the mind only conceives of one object at a time". However, the subjective time notion of now does have a temporal extension. According to my reading on the subject, most cognitive scientists and philosophers place a value of between two and three seconds of subjective time on the "now". In my experience, the subjective now can be shorter in duration depending on the circumstance.
Consider the example where one if faced with a life threatening situation. I was once climbing a rock face without any safety equipment when I started to lose my balance. Within the subjective now - i.e., in no more that one second - I had to conceive of many objects in my environment as well as many abstract objects. I was able to make the decision to lower my centre of gravity in order to regain my footing. After taking appropriate action, all that was left for me to do was to clean my underpants.
Your questions asks about the processing capacity of our mind. Apparently, this capacity is highly limited, it seems that there is no parallel processing by the mind.
We can achieve several conscious operations in a short time only by interleaving, allocating to each operation again and again a short timeslice.
I do not think that the grammar of the natural language helps much to understand why our mind operates this way. Take the sentence "The brown dog is barking." Literally it has one predicate "is barking", but it indicates two properties "brown" and "barking" which we recognize at the same time. In addition, there is a difference between the capacity to recognize, which uses many unconscious processes, and the capacity to think, which are conscious mental processes.
The human mind is capable of parallel processing and does so on a regular basis. Although language is limited by its structure and gives the impression that we can only process predicates sequentially, if we look at the way we perceive objects in images or process incoming sounds, it is obvious that we can process multiple predicates simultaneously. Indeed Jerry Fodor's language of thought hypothesis was criticized for exactly this reason, that it didn't take into account the parallel processing that humans seemed capable of.
When I look around this room where I am right now, I grasp it all at once; I see many things, but my attention is not focused on any one thing; as my mind is on writing this post.
And what I am writing, is made of many things - it's sense, the sentence, the words, the letters; that the letters are made of lines ...
In Kantian language, there is a unity of apperception that grasps the manifold of many intuitions, sensations and concepts understood together in a single synthesis; and this in a subjective and subject-filled moment of time.
What we grasp, all at once, is a whole made of many parts; so, yes - we do think of 'one thing at a time' - but it's better thought of a whole, whose unity of parts changes in time; it expands, it diminishes; and in some places, being more focused - in others less.
It's this picture, that informs the stream of consciousness technique, in early modernist writing: Joyce, Faulkner or Woolf.
The conciousness is etymologically ''the knowing of''. From the dhamma, yes, there is conciousness of one thing at ''a time''. see the dependant origination : https://en.wikipedia.org/wiki/Prat%C4%ABtyasamutp%C4%81da https://en.wikipedia.org/wiki/Idappaccayat%C4%81