.map().... is there an upper limit? ...and i'm sure i've asked before, but there's no way of batching it so say like 5 run at a time?

The limit is as much as your infrastructure can handle 🙂 I’ve seen one person from the community separated a flow of 8k child tasks into 8 flow runs (each running 1000 child tasks in parallel) based on various parameter values:

from prefect import Flow, task, unmapped
from prefect.tasks.prefect import create_flow_run
from prefect.executors import LocalDaskExecutor


@task
def generate_thousand_numbers(start, stop, step):
    nrs = range(start, stop, step)
    return list(nrs)


with Flow("mapped_flows", executor=LocalDaskExecutor()) as flow:
    flow_run_1 = generate_thousand_numbers(1, 1000, 1)
    flow_run_2 = generate_thousand_numbers(1000, 2000, 1)
    flow_run_3 = generate_thousand_numbers(2000, 3000, 1)
    flow_run_4 = generate_thousand_numbers(3000, 4000, 1)
    flow_run_5 = generate_thousand_numbers(4000, 5000, 1)
    # ... until 8
    parameters = [
        dict(list_of_numbers=flow_run_1),
        dict(list_of_numbers=flow_run_2),
        dict(list_of_numbers=flow_run_3),
        dict(list_of_numbers=flow_run_4),
        dict(list_of_numbers=flow_run_5),
        # ... until 8
    ]
    mapped_flows = create_flow_run.map(
        parameters=parameters,
        flow_name=unmapped("dummy-child-flow"),
        project_name=unmapped("community"),
    )