There are a number of ways to find your project ID.
If you are using the OpenStack command line tools you have most likely sourced an openrc file, as explained in Command line interface (CLI). If this is the case then you can find your project ID by issuing the following command:
$ echo $OS_TENANT_ID 1234567892b04ed38247bab7d808e214
Alternatively you can use the openstack configuration show command:
$ openstack configuration show -c auth.project_id -f value 1234567892b04ed38247bab7d808e214