Every organization is different when it comes to their data and what needs to be brought over during a cloud migration project. Although we don't recommend bringing over more than a year's worth of historical data, some may require more than that for certain business processes or legal requirements, so it's important to take stock of your data before getting started so that you can avoid the transfer of unnecessary data.
It's not just about knowing what master data you need on the new system; something even more important is ensuring that the data is clean before you ever kick off. Although this will take a lot of work, you will reap the benefits of having a pristine house at the conclusion of the implementation. It will also help ease your transition into using a cloud-based platform.
Something else I want to address about data is having enough time for conversions. When we get to the first cycle of these during a project, it is always full of data surprises that impact a timeline. Keep this in mind when putting together an estimated timeline and schedule as much time as is feasible around this phase - it's always better to be ahead of schedule than behind!