Administrators are able to increase the limit from 20 snapshots globally in the deployment or to authorize individual projects to ignore these snapshot limits, both of those options are outlined here: https://docs.dominodatalab.com/en/latest/admin_guide/902e0a/datasets-administration/#_set_limits_on_datasets_usage
To keep these limits in place for the project and still avoid this error, snapshots can be marked for deletion: https://docs.dominodatalab.com/en/latest/user_guide/0a8d11/datasets-overview/#_manage_datasets_and_snapshots
And deleted by an Administrator: https://docs.dominodatalab.com/en/latest/admin_guide/902e0a/datasets-administration/#deleting-snapshots-from-datasets
In addition, for version 3.6+ there is the option to delete snapshots using the Domino API: https://dominodatalab.github.io/api-docs/#/reference/datasets/snapshot-operations/archive-snapshot
To get the list of snapshot IDs for each dataset, you can use the API for retrieving dataset details: https://dominodatalab.github.io/api-docs/#/reference/datasets/dataset-operations/get-dataset-details. It would be possible to set up a scheduled run using these API calls for deleting snapshots on a regular basis, though you'll want to be very careful with this operation to avoid losing critical data.