Can I just link one of the many online banking account to QuickBook?

Yes, you can connect one bank account from the multiple ones in QuickBooks, kl1340. It'll be my pleasure to guide you through the process.

If this is your first time, you'll see the Connect account button while you can Link accounts if there are existing ones. Follow the steps below:

1. Go to the Transactions menu, then select Bank Transactions.
2. Select Connect account or Link Account.
3. Enter the URL or name of your bank in the Search field, then choose the bank.
4. Type your Sign info in the Login and Password fields, then click Continue. 
5. Pick the account you want to connect and date to pull transactions from the dropdown ▼. Then, select Next.
6. Determine your account classification in the Account type ▼ dropdown, then tap Next.
7. Follow the onscreen instructions.

Some banks let you download the last 90 days of transactions, while others can go back 24 months. If you'd like to add another one, check out this article: Connect bank and credit card accounts to QuickBooks Online.

Once connected, the system automatically imports the latest transactions and suggests categories. Refer to this article for complete details: Categorize online bank transactions in QuickBooks Online.

Once everything's settled, I recommend reconciling regularly to ensure your account is balanced and accurate. Save this guide for future reference: Reconcile an account in QuickBooks Online.

For your additional concerns about matching bank transactions or classifying accounts, you can always get back to this thread. I'll always be right here to assist you.