I would like to know How many envelopes available in to my account?

It depends on your Account plan as to the number of envelope. Here is a article and FAQ that describes some features of basic account plans...

https://www.docusign.com/products-and-pricing

"The number of document sends included each month differs by plan type. DocuSign’s Personal plan includes 5 document sends per month. DocuSign Standard, Real Estate & Business Pro plans purchased online do not have a hard usage cap, but are subject to reasonable use limits."

I already have DocuSign account and we purchase 4000 envelops now how do I know how many envelops available in to my account?

The answer is you can't see how many are left. Docusign purposely makes this hard to find, and clearly doesn't answer your question. They answer how to see how many you've used, while going out of their way to ignore your question how many are left.

@Affiliated Docusign​ The link I provided on this thread clearly shows how to locate that information on Free, Personal or Individual accounts. Also with higher end accounts if you have a certain number of envelopes available and you run a simple Envelope Report on how many are used, then using math should allow you to simply calculate the remaining envelopes.

https://support.docusign.com/en/guides/ndse-admin-guide-billing-and-usage-information

@David Schmitz​ 

Note: Only free, individual, or personal accounts have access to "View Usage Details". This shows how many you have available and how many you've used. If you have a Business account, then you would have to know how many envelopes came with your plan and then run a usage report to figure out how many you have remaining.