calculating the 'serial'?

I'm trying to calculate a serial number and having trouble. I'm unsure
of the jinja filter pipeline. Here's what I'm trying:

serial: "{{play_hosts|length|float/2|round(0, 'ceil')|int}}"

here's the error:

ValueError: invalid literal for int() with base 10:
"{{default(length(play_hosts), 1)/2|round(0, 'ceil')|int}}"

If I put that jinja string in a debug, it is ending with a float- for
instance, if there are 10 play_hosts, the output is 5.0. That seems
strange given the 'int'. I wonder if that's what is causing it to
fail.

-ted

It appears as though serial cannot be templated. Note that you can use a percentage there already. So just specify 50% and I think that should match what you want.

https://docs.ansible.com/playbooks_delegation.html#rolling-update-batch-size