I deleted an invoice and when I started a new invoice, that invoice number is missing which I deleted. How can i get the same invoice number which I deleted?

Good to have you in the Community space, @tejinder1.

QuickBooks Online remembers the invoice number and continues the sequence. Thus, creating a new one will automatically populate to the next number order.

What we can do is to edit the number of your invoice to the one you've deleted. I'll show you how. 

1. Go to the + New menu in your QuickBooks Online (QBO).
2. Select Invoice.
3. Go to the Invoice no. field and change the invoice number from there. 
4. Enter the needed details of the invoice and click Save and close or Save and new once done.

You can also check this link to learn more about invoice numbers in QBO: How to change the invoice number on sales forms.

In case you want to learn how to modify your invoices, you can refer to the steps outlined in this article: Customise invoices, estimates, and cash memos in QuickBooks Online.

I'm all ears if you need more help with your invoices and QBO. It's always my pleasure to help you run your business seamlessly. Have a great day and keep safe!