One of the fundamental differences between traditional waterfall delivery and agile delivery is planning. For the former, planning is performed upfront with an entire view of scope and an end date, typically defined by management. For agile, planning of performed iteratively and continuously assessing the need to adapt to emerging needs and changes. 
The common practice is to use velocity to determine capacity.
Velocity-based planning is based on the premise that the amount of work a team will do in an upcoming Sprint is roughly equal to what they have done in prior Sprints.  Various assumptions are made such as a constant team size, similar type of work, consistent Sprint lengths, etc.To utilise velocity-based planning the team needs to have base line data to form assumptions around the anticipated future velocity of the team. Hence, Story Point estimation plays a greater role.
Velocity changes over time, so ensure you don’t use:
Be aware that planning out an entire quarter introduces significant uncertainty, even if just from the perspective of natural variability. The futher our the team forecasts their load versus capacity the mor elikely their estimations will be inaccurate.
Ensure that the forward plan for Sprints is adjusted each Sprint Review and expectations set with stakeholders and the Product Owner regarding what can be delivered and when.
People are tempted to ensure each Sprint is "full" so that the team is "fully utilised". When a whole quarter of Sprints is full, there is no room to take on feedback at the end of each Sprint. Ensure that there is some capacity to take on feedback and adapt each Sprint. If a Sprint isn't full, but the team finishes early, then it can simply take on more work directly from the Product Backlog.
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