I got you covered in maintaining your clients in QuickBooks Online (QBO), @umang232-gmail-c.
With your QBO single subscription, you're able to add and manage an unlimited number of clients (customers). This is because QuickBooks make sure the system keeps up with your growing business and help you stay organised.
To keep your client list updated, you can refer to this article: Add and manage customers in QuickBooks Online. It contains steps about adding customers/sub-customers, deleting them, and merging duplicate ones.
Also, QBO provides a report (Customer Contact List) where you can view your client details (i.e., phone number, email, billing address, and other contact info). Just visit the Sales and customers section from the Report menu's Standard tab.
Additionally, here's an article that'll further guide you in effectively managing your clients and income transactions using QBO: QuickBooks Help Articles. It includes topics about invoices, viewing transactions, and pulling up reports, to name a few.
In case you have other concerns about managing your clients in QBO, I'll be right here to help. Take care and stay safe always.