I answered this question already this morning, but because our hosting provider experienced some troubles with the MySQL server my reply was lost. Actually we are running on a backup of the database from 15h ago.
Anyway. Here is what I wrote this morning.
We are adding those number because we can run into duplicate aliases. And if we have duplicate aliases then we won't have the right urls to articles.
If you are absolutely sure that you won't run into this problem you can go to:
administrator/components/com_cmigrator/library/migrators/com_content/wordpress.php (if you use the k2 migrator then check it in the com_k2 folder)
on line 203:
Now import again and your content should not have the number in the alias. However as Robert said - joomla will most probably add the article-id in front of the alias... And as Robert said -> you can control this stuff with component such as sh404 or joomSef or other well known components
Cheers and hopefully our db won't experience another meltdown today
Let us say that you are writing an article with the title "Something". Next week you write another article and call it again "Something" -> when you try to save your article joomla will refuse to do so and will tell you to think of another alias for it. That is why I needed to ensure that when we import the articles we have unique aliases.