You can message multiple contacts at once from the Inbox.
For ongoing or triggered outreach, newer tools are usually a better fit: use SMS Marketing or Email Marketing for campaigns to opted-in contacts, and Automations (Engagement › Automations) to send automatically when a contact takes an action. Bulk one-off sends remain available for quick, manual messages.
See "SMS Marketing: Getting Started," "Email Marketing: Getting Started," and "Automations — Getting Started." The same consent/opt-in and 10DLC rules apply to all outbound messaging.