Would it be possible to know the number of Bitcoin addresses that has at least been used say 10 times if so maybe it's possible to get an educated estimate no?

The actual number of people using bitcoin is not possible to determine.

Even if you track the number of accounts, then also it is not possible because, one user can generate lot of accounts.

Only if you can track the IP of the users then you might get an estimate. But there's no easy way to do it.

Honestly you can not easily know how much people are using bitcoin because its anonymous and many addresses are generating everyday.. and also many people are using multiple wallet for security reason..  so for me its impossible to determine how much the actual number of people using bitcoin..

I donot think so. it will take much days for us to calculate the no. of addresses used in the past few say only days. forget about years.

Where is your country?

Ask some of the regulated exchanges to publish their updated user totals for 2016. The regulated exchanges links real identities to their users with verified documentation. Then extrapolate numbers from that with updated information from public wallet providers, like Blockchain.info. You will never get a 100% correct answer, but this will be accurate enough.

Get in touch with Xapo Circle Coinbase etc. ^smile^

My guess based on circle, blockchain and coinbase wallet download data and discounting to allow for multiple wallets by same holder,  would be perhaps ~8-10mn holders, out of whom maybe several hundred thousand are really active

It is not known exactly. I tried to search for information and found only for 2014 year - "virtual" currency of the Bitcoin uses more than 40 million people."
I think now, in connection with the trust and prevalence, users much more.

PS
I found an interesting site http://getaddr.bitnodes.io/
Ranking of countries on whose territories are already using Bitcoins. This site displays in real time the number of people using Bitcoin client. 
When you select a specific country are displayed around the city distribution by IP addresses of people.

If what you are saying about total number of transaction being only a few million than the number of people using bit coin can't really be even a million. It's amazing how much publicity it has received though.