I would forecast the needs of the department based on worst case scenario of overlapping absences of business-critical positions and require redundancy in the tema members to ensure full coverage of all work.
I am currently the director of engineering for a multibillion dollar company and have acted in the capacity of starting engineering organizations from the ground up and building them to the point of critical mass for the successful integration into the company. You have told me that your organization is in a growth mode and that is the type of organization I am experienced at planning and developing strategy for.
The difference in leading ten and leading twenty isn't in how you lead, but in which other tasks you need to prioritize-down to have the time to lead the additional engineers. Alternatively to prioritizing things off of my lap, I can mentor and coach teammates into most of those tasks over time; I've shown this in my current role by repeatedly coaching individual contributors into leads, and doing so successfully enough to make my team a destination inside the company for talent to transfer *to* instead of transfer *from*.