  DNA labeling is a tool in molecular biology and biotechnology to visualize,
detect, and study DNA at the molecular level. In this process, a DNA molecule
is labeled by a set of specific patterns, referred to as labels, and is then
imaged. The resulting image is modeled as an $(\ell+1)$-ary sequence, where
$\ell$ is the number of labels, in which any non-zero symbol indicates the
appearance of the corresponding label in the DNA molecule. The labeling
capacity refers to the maximum information rate that can be achieved by the
labeling process for any given set of labels. The main goal of this paper is to
study the minimum number of labels of the same length required to achieve the
maximum labeling capacity of 2 for DNA sequences or $\log_2q$ for an arbitrary
alphabet of size $q$. The solution to this problem requires the study of path
unique subgraphs of the de Bruijn graph with the largest number of edges and we
provide upper and lower bounds on this value.
