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The small-signal stability of multi-terminal high voltage direct current (HVDC) systems has become one of the vital issues in modern power systems. Interactions among voltage source converters (VSCs) have a significant impact on the stability of the system. This paper proposes an interaction quantification method based on the self-/en-stabilizing coefficients of the general <tex>$\boldsymbol{N}-\mathbf{terminal}$</tex> HVDC system with a weak AC network connection. First, we derive the explicit formulae of self-/en-stabilizing coefficients for any <tex>$\boldsymbol{N}-\mathbf{terminal}$</tex> HVDC system, which can quantify the interactions through different paths analytically. The relation between the self-/en-stabilizing coefficients and the poles of the system can be used to evaluate the impact of the interactions on the system stability effectively. Then, we employ the obtained formulae to analyze the parameter sensitivity and explain how a parameter affects the stability of the system through different paths of interactions. Finally, extensive examples are given to demonstrate the effectiveness of the proposed method.