Is your product team stable?
Velocity is a metric to measure the progress of the work completed in one iteration. Usually, the unit is expressed in Story Points.
Velocity is a rate at which Product Backlog Items are finished and accepted by the Product Owner. Partially completed PBIs are not accounted for, and there are several good reasons for that.
Usually, it takes several iterations in a team with a constant number of members for velocity to settle down. If it doesn't, somebody, usually the scrum master, must bring this to attention. If you can not keep the velocity in pre-determined margins, it may be better for the team and the organization to practice Kanban instead.
If the velocity is stable, you can predict when a certain amount of Product Backlog Items will be releasable. For that, you should have enough PBIs in the Definition of Ready state in the Product Backlog, but be careful, as too much upfront planning is not advisable. Make sure you have versioning in place, so you can track your progress by version increments, which also helps with planning.
Find out if Scrum is even suitable for this team. If you can not stabilize Velocity in a few months, then you should consider using other agile frameworks, like Kanban to drive your development effort.