Yes- I totally agree!
Most of the times we see this in startups.
With my experience i have seen too and to manage this properly -
1. I understand the Business AOP (Annual Operation Plan)and follow that closely.
2. Sometime there are slight changes that comes with addition or reduction in requirements because of change in Market, but since we are always taking exploratory interviews , we never find shortage of pipeline.
3. To Make sure our leaders understand the business expansion properly we ask them to do back calculations with respect to targets.( People who will impact your monthly numbers directly and those who will be in the support roles like- ops, hr ,finance,marketing, program management etc)there is a ratio that every business follows.
From HR side how we support TL in forecasting the resource requirement-
We speak to the entire team individually during Hr 1-1 and nudge them and understand the KPI, work division( if resources are under utilised or over utilised) and time they take to complete the task and then draw mean median and max graph for the entire team ( work vs time) and then identify weather we have surplus or deficit of resource and share the data with BU heads to take decision based on data.
while calculating you have to be mindful of attrition rate in the team, hiring speed, time taken by new joiners to scale up/start performing working day/hr for that month etc).