Exact meaning of "degree of crosslinking" in polymer chemistry
Normally, in practice the amount fraction of DVB in the feed is used as a nominal number for the degree of crosslinking. IUPAC provides a definition in a somewhat unexpected place [1, p. 2352]:

Crosslinking: Property of a solid support prepared from polymeric materials with interconnected strands. Often results from the inclusion of multifunctional monomers in the polymerization reaction, e.g. divinylbenzene in polystyrene production. In such cases, the degree of crosslinking is often quoted as the proportion of the multifunctional monomer in the reaction mixture. The extent of crosslinking is important for physical properties of the solid support, such as the propensity to swell in different solvents [18].

Following reference [18], one can find further clarification [2, p. 2278]:

Care is therefore required in defining or interpreting the DVB content of crosslinked polymers since this might be quoted as a percentage of technical DVB used to make the
polymer, or the figure can be adjusted to reflect only the content of actual DVB isomers present. Since for convenience the nominal crosslink ratio or degree of crosslinking of a polymer network is often quoted as the mol% of crosslinker used to prepare the network, defining the actual percentage of DVB isomers employed is more informative since this equates with the nominal crosslink ratio (Fig. 4). Bearing in mind that there is no unambiguous method, and certainly no simple and rapid method, for determining the real crosslink ratio in a polymer network, the nominal figure based on the actual DVB feed is a
very useful parameter.

Fig. 4 Polymerisation of styrene and divinylbenzene to form an infinite polymer network.

Further, the paper lists several reasons as to why the real level of crosslinking is always accompanied with a level of uncertainty:

The effective crosslink ratio significantly decreases with greater quantity of DVB being fed due to growing number of unreacted vinyl groups.
Entanglement crosslinking occurs at higher rates of polymerization in slowly-agitated systems and results in additional mobile crosslinking.


Fig. 5 Permanent entanglement crosslink


Additional crosslinks can also be created as the result of intramolecular side-reactions.

These factors are tricky to estimate quantitatively; a method allowing to do so would be a single pulse excitation (SPE) 13C solid state NMR.
References

Maclean, D.; Baldwin, J. J.; Ivanov, V. T.; Kato, Y.; Shaw, A.; Schneider, P.; Gordon, E. M. Glossary of Terms Used in Combinatorial Chemistry. Pure and Applied Chemistry 1999, 71 (12), 2349–2365. DOI: 10.1351/pac199971122349.
Sherrington, D. C. Preparation, Structure and Morphology of Polymer Supports. Chem. Commun. 1998, No. 21, 2275–2286. DOI: 10.1039/a803757d.