I have created new blank site in kentico CMS 7 and in the new site i have created a blank page using ad-hoc page template and in the design tab, template wizard is not shown .
When you created your new site it sounds as if you chose Blank ASPX site vs Blank Portal site. If that's the case the design tab is not available for an ASPX site. Here's some documentation regarding the differences. I'd highly recommend Portal. I use portal on all my sites and have integrated with many 3rd party systems without issue.
but in the case of customization which is the better. aspx+portal or portal???
Depends on what you're customizing. I've been working with Kentico since v4 and haven't had a need yet to use ASPX or a combination. Learn how to create modules, webparts and widgets and you'll have no need.
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