how we can buy photos?

You can either take a subscription or buy credit packs. Subscriptions are available on the month (pricier, but cancellable anytime without a fee) or as an annual subscription paid on the month (you agree to continue your subscription for a year). Credit packs do not need a subscription.

Each asset (except for premium, and videos) costs one credit. Other types of assets can be bought by using more credits. The number of credits required depends on your subscription.

For a website, you won't need extended licences.

See here for more information: https://stock.adobe.com/plans