For Foucault, the relations of subordination within which we live actually create and enable our ability or agency to resist those same relations of subordination/domination. This is why he raises the question of limits here.
It may be helpful to consider Saba Mahmood’s summary of Foucault on “subject formation” in Politics of Piety, p. 17: “Power, according to Foucault, cannot be understood solely on the model of domination as something possessed and employed by individuals or sovereign agents over others, with a singular intentionality, structure, or location that presides over its rationality and execution. Rather, power is to be understood as a strategic relation of force that permeates life and is productive of new forms of desires, objects, relations and discourses (Foucault 1978, 1980). Secondly, the subject, argues Foucault, does not precede power relations in the form of an individual consciousness, but is produced through these relations, which form the necessary conditions of its possibility. Central to his formulation is what Foucault calls the paradox of subjectification: the very processes and conditions that secure a subject’s subordination are also the means by which she becomes a self-conscious identity and agent…” So the abilities that define a subject’s modes of agency are “not the residue of an undominated self that existed prior to the operations of power but are themselves the products of those operations.”