I think you have in mind the following passage from the Grundrisse, from the section on "The Method of Political Economy” - the line you’re paraphrasing is in the middle of the second paragraph, and is translated here as "The concrete is concrete because it is the concentration of many determinations”:
"When we consider a given country politico-economically, we begin with its population, its distribution among classes, town, country, the coast, the different branches of production, export and import, annual production and consumption, commodity prices etc.
"It seems to be correct to begin with the real and the concrete, with the real precondition, thus to begin, in economics, with e.g. the population, which is the foundation and the subject of the entire social act of production. However, on closer examination this proves false. The population is an abstraction if I leave out, for example, the classes of which it is composed. These classes in turn are an empty phrase if I am not familiar with the elements on which they rest. E.g. wage labour, capital, etc. These latter in turn presuppose exchange, division of labour, prices, etc. For example, capital is nothing without wage labour, without value, money, price etc. Thus, if I were to begin with the population, this would be a chaotic conception [Vorstellung] of the whole, and I would then, by means of further determination, move analytically towards ever more simple concepts [Begriff], from the imagined concrete towards ever thinner abstractions until I had arrived at the simplest determinations. From there the journey would have to be retraced until I had finally arrived at the population again, but this time not as the chaotic conception of a whole, but as a rich totality of many determinations and relations. The former is the path historically followed by economics at the time of its origins. The economists of the seventeenth century, e.g., always begin with the living whole, with population, nation, state, several states, etc.; but they always conclude by discovering through analysis a small number of determinant, abstract, general relations such as division of labour, money, value, etc. As soon as these individual moments had been more or less firmly established and abstracted, there began the economic systems, which ascended from the simple relations, such as labour, division of labour, need, exchange value, to the level of the state, exchange between nations and the world market. The latter is obviously the scientifically correct method. The concrete is concrete because it is the concentration of many determinations, hence unity of the diverse. It appears in the process of thinking, therefore, as a process of concentration, as a result, not as a point of departure, even though it is the point of departure in reality and hence also the point of departure for observation [Anschauung] and conception. Along the first path the full conception was evaporated to yield an abstract determination; along the second, the abstract determinations lead towards a reproduction of the concrete by way of thought. In this way Hegel fell into the illusion of conceiving the real as the product of thought concentrating itself, probing its own depths, and unfolding itself out of itself, by itself, whereas the method of rising from the abstract to the concrete is only the way in which thought appropriates the concrete, reproduces it as the concrete in the mind. But this is by no means the process by which the concrete itself comes into being. For example, the simplest economic category, say e.g. exchange value, presupposes population, moreover a population producing in specific relations; as well as a certain kind of family, or commune, or state, etc. It can never exist other than as an abstract, one-sided relation within an already given, concrete, living whole. As a category, by contrast, exchange value leads an antediluvian existence. Therefore, to the kind of consciousness – and this is characteristic of the philosophical consciousness – for which conceptual thinking is the real human being, and for which the conceptual world as such is thus the only reality, the movement of the categories appears as the real act of production – which only, unfortunately, receives a jolt from the outside – whose product is the world; and – but this is again a tautology – this is correct in so far as the concrete totality is a totality of thoughts, concrete in thought, in fact a product of thinking and comprehending; but not in any way a product of the concept which thinks and generates itself outside or above observation and conception; a product, rather, of the working-up of observation and conception into concepts. The totality as it appears in the head, as a totality of thoughts, is a product of a thinking head, which appropriates the world in the only way it can, a way different from the artistic, religious, practical and mental appropriation of this world. The real subject retains its autonomous existence outside the head just as before; namely as long as the head’s conduct is merely speculative, merely theoretical. Hence, in the theoretical method, too, the subject, society, must always be kept in mind as the presupposition."
Here’s a direct link to the beginning of the relevant section: https://www.marxists.org/archive/marx/works/1857/grundrisse/ch01.htm#3