If you've got a free Substack publication, you can start from anywhere. If you're planning to add paid subscriptions, you can view a list of countries supported by Stripe, our payments provider.
Please note that India is an exception to this list for additional technical reasons. Unfortunately, we do not support paid subscriptions for writers in India at this time. (Note that this restriction does not apply to India-based customers of your publication, but to India-based writers who would like to set up paid subscriptions.)