Blogger shows by default 20 posts per page when you click specific label from your labels widget. But maybe you want to show less because page loads too long... or you want to show maximum (which is 100)?!

Go to Dashboard > Layout > Page Elements > Edit Html > Expand Widgets

Search for this:

<a expr:dir='data:blog.languageDirection' expr:href='data:label.url'>
<data:label.name/>
</a>
</b:if>
<span dir='ltr'>(<data:label.count/>)</span>

And replace it with this:

<a expr:dir='data:blog.languageDirection' expr:href='data:label.url + "?max-results=5"'>
<data:label.name/>
</a>
</b:if>
<span dir='ltr'>(<data:label.count/>)</span>

Note: the red "5" determins number of your posts before Blogger shows "Read older posts" link. It could be anything between 1 and 100.