How to insure support for theme users ?

I am coding a wp theme to be published in themeforest, Is there a ready solution to insure support for my future clients ?

Considering that you will be the one providing the support, it is up to you to implement whatever system you think works best for you. You can either set up your own support system (for example, via WP; there are matching themes and plugins for that available on Envato), use a PHP only system (again, there are scripts for that available on Envato as well), or use any of the cloud based offers (i.e. Ticksy, or others like it). Either way, you are the one responsible for ensuring that your clients receive support.

Thank you very match for your answer.
Isn’t there a basic free solution ?

As with most things in life, you get what you pay for (or don’t pay for). Providing support to your item(s) can take up a fair amount of time, the more sales you have, and you want a system that is reliable, easy to access for your buyers, and can fulfill all your requirements (license check, notifications, attachments, etc.). And those things usually cost money. There simply is more to being an author/seller than just uploading an item to Envato and waiting to collect money from your sales.

Ok, thank you for your help