If i buy Envato Elements Subscription then do i get the access to all wordpress themes in Envato marketplace?
No - you get WP from Elements Download WordPress Themes & Templates - Envato Elements
Bear in mind that Elements does not come with support or updates that are included with Themeforest and full purchases
Means Envato Elements Subscription does not include WP themes access. Correct?
From where I get access to all WP themes or do in need to buy each theme separately.
You get all the WP themes in that link Download WordPress Themes & Templates - Envato Elements - you do not get every theme on Themeforest.
Some themes will be available on both marketplaces and some will only be available on either Elements or Themeforest