I can't find it in the docs, but there has to be a maximum. In a salesforce flow, what is the maximum number of records a fast lookup can return. What happens if the number is higher than the max?
I just tested this, and there's no limit for the number of rows a Fast Lookup can return directly, but you are bound by the governor limits in Apex Code and Visualforce: heap size, query row limits, query limits, view state size, and CPU time. Attempting to update those records may also run into other limits, such as the DML statement limit and the DML total record limit (plus any of the previously mentioned limits).