As power systems transition toward inverter-based resources (IBR)-dominated grids, Grid-ForMing (GFM) IBRs are increasingly expected to provide the voltage-source characteristics required to maintain adequate system strength. However, the quantification of how much GFM capability is required and where it should be located in the grid remains unclear. This paper investigates the impact of the amount and location of GFM IBRs on small-signal system strength. The Jacobian Voltage Stiffness Metric (JVSM), which quantifies the voltage-magnitude and voltage-phase stiffness of the grid from its frequency-domain Jacobian characteristics, is used as a bus-wise system-strength measure. Unlike existing metrics, JVSM captures the relevant frequency-dependent dynamics at each grid location without assuming ideal voltage-source behaviour or requiring detailed models, reference impedances, or computationally intensive, error-prone eigenvalue or singular-value analyses. The impact of GFM provision is systematically assessed by varying the number and location of GFM IBRs in the grid. The analysis on a modified IEEE 39-bus system demonstrates that increasing the number of GFM IBRs generally improves system strength; however, JVSM quantifies variations in bus-wise system strength and characterises their dependence on GFM location, with the strength assessments validated through small-signal modal analysis. The results show that specifying GFM requirements solely in terms of penetration level or installed capacity may not adequately characterise the resulting system strength. This quantification provides a basis for the future development of GFM requirements that account for both the amount and location of GFM capability.