$ juju set-model-constraints --help
Usage: juju set-model-constraints [options] <constraint>=<value> ...

Summary:
Sets machine constraints on a model.

Options:
-m, --model (= "")
    Model to operate in. Accepts [<controller name>:]<model name>

Details:
Sets machine constraints on the model that can be viewed with
`juju get-model-constraints`.  By default, the model is the current model.
Model constraints are combined with constraints set for an application with
`juju set-constraints` for commands (such as 'deploy') that provision
machines for applications. Where model and application constraints overlap, the
application constraints take precedence.
Constraints for a specific application can be viewed with `juju get-constraints`.

Examples:

    juju set-model-constraints cpu-cores=8 mem=16G
    juju set-model-constraints -m mymodel root-disk=64G

See also: models
          get-model-constraints
          set-constraints
          get-constraints