The short answer
From the agency view, start a new sub-account and fill in the business details, or create it straight from a snapshot under Account Snapshots with the Create New Account button so the template loads during setup. You need agency admin access. The Starter plan allows 3 sub-accounts; Unlimited and Agency Pro have no cap.
How many sub-accounts your plan allows
A sub-account is the separate workspace for one business, with its own contacts, calendars, funnels, phone numbers and workflows. HighLevel's pricing page gives the Starter plan ($97 a month) 3 sub-accounts, and the Unlimited ($297) and Agency Pro ($497) plans unlimited sub-accounts. Agency Pro adds SaaS Mode and automated sub-account creation, where a client pays through your checkout and HighLevel creates their account. Contacts and users are unlimited on every plan, so on Starter the sub-account count is usually the first limit an agency reaches.
Create a sub-account from a snapshot
HighLevel documents this route step by step. From the Agency Dashboard, click Account Snapshots in the left menu, find the snapshot you want as the starting point and click the blue Create New Account (+) button beside it. Fill in the new sub-account's details and click Add Sub-Account. The snapshot loads automatically, so its funnels, forms, workflows, calendars and settings are there from the first login. Only one snapshot can be applied during creation, though more can be loaded later. Funnels and websites arrive as drafts and stay offline until you connect a domain and publish them.
Choose the account type and finish setup
When you create a sub-account, HighLevel may ask what type it is. Choose This is my client's account for a client's business, or This is my own account for your agency's own workspace or SaaS and rebilling setup. The second option makes it an Agency Sub-Account, marked with a blue badge, which cannot be transferred to another agency. A loaded snapshot cannot be removed as a whole later, so pick it with care. Typical next steps inside a new account are buying a phone number and starting A2P registration, setting up email sending, building calendars and inviting the client's users.
More answers in HighLevel questions. For costs, see GoHighLevel pricing or run the cost calculator.