Stoichiometry-controlled structural order and tunable antiferromagnetism in Fe <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi/> <mml:mi>x</mml:mi>

Transition metal dichalcogenides (TMDs) enable magnetic property engineering via intercalation, but stoichiometry-structure-magnetism correlations remain poorly defined for Fe-intercalated $\mathrm{NbSe_2}$. Here, we report a systematic study of $\mathrm{Fe}_{x}\mathrm{NbSe_2}$ across an extended composition range $0.05 \le x \le 0.38$, synthesized via chemical vapor transport and verified by rigorous energy-dispersive x-ray spectroscopy (EDS) microanalysis. X-ray diffraction, magnetic, and transport measurements reveal an intrinsic correlation between Fe content, structural ordering, and magnetic ground states. With increasing $x$, the system undergoes a successive transition from paramagnetism to a spin-glass state, then to long-range antiferromagnetism (AFM), and ultimately to a reentrant spin-glass phase, with the transition temperatures exhibiting a nonmonotonic dependence on Fe content. The maximum N\'eel temperature ($T_{\mathrm{N}}$ = $\mathrm{175K}$) and strongest AFM coupling occur at $x=0.25$, where Fe atoms form a well-ordered $2a_0 \times 2a_0 $ superlattice within van der Waals gaps. Beyond $x = 0.25$, the superlattice transforms or disorders, weakening Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions and significantly reducing $T_{\mathrm{N}}$. Electrical transport exhibits distinct anomalies at magnetic transition temperatures, corroborating the magnetic state evolution. Our work extends the compositional boundary of Fe-intercalated $\mathrm{NbSe_2}$, establishes precise stoichiometry-structure-magnetism correlations, and identifies structural ordering as a key tuning parameter for AFM. These findings provide a quantitative framework for engineering altermagnetic or switchable antiferromagnetic states in van der Waals materials.