In order to study the ralationship between grid-forming （GFM） and grid-following （GFL） units that can support the stable operation of renewable systems, this paper firstly takes the single wind farm integrated system as an example to analyze the dynamic characteristics of the GFL and GFM units. Then, from the perspective of maintaining the oscillation stability of wind farms both in low frequency and subsynchronous frequency bands, the calculation principle of the GFM unit capacity to be deployed in wind farms is proposed. On this basis, based on the three-machine-nine-bus system, the feasibility of the stable operation of the 100% renewable system is explored, and the influence of the proportion and location of the GFM units on the dynamics of the system are studied. The results show that the subsynchronous oscillation （SSO） risk can be effectively improved by deploying a small number of GFM units in the wind farm, and the capacity of GFM units is mainly constrained by the low-frequency mode stability. Besides, in the 100% renewable system, the SSO due to inadequate support of GFM to the remote GFL units is the key constraint in determining the need for GFM units.