Posts number in the blog can be controled through the blog dashboard using an option called Max posts shown on main page, unless you have a custom theme that has its own option.
The problem is That
Sometimes you set the number to be for example 7 but you just see only 2, 3 or 5 posts in the homepage and other pages but not always 7 posts as it should do.
The Cause
Number of posts is controlled by Blogger using Auto Pagination system. It displays up to a maximum of ~1MB of posts. If this maximum quota is not reached, the number of posts displayed will depend on the the number set in the Max posts shown on main page option. If not, posts that are over the quota limit will be automatically moved to the next page.
The Solution is Simple
To fix it, you need to insert a jump break in your posts just after the first image and a few lines from the beginning as in the following image.
the jump break will look like this <!--more--> in post HTML view.
If you have any questions, let me know in the comments.