Hm… I don’t know if there’s a better way to do this. If someone comes along and says there is, I’d use her or his method over this one since this involves editing the core.
But, here goes:
You need to edit 3 lines of wp-includes/template-functions-general.php
line 206
change
global $querystring_start, $querystring_equal, $querystring_separator, $month, $wpdb;
to
global $querystring_start, $querystring_equal, $querystring_separator, $month, $month_abbrev, $wpdb;
line 252
change
$text = sprintf("%s %d", $month[zeroise($arcresult->month,2)], $arcresult->year);
to
$text = sprintf("%s %d", $month_abbrev[$month[zeroise($arcresult->month,2)]], $arcresult->year);
line 255
change
$text = sprintf("%s %d", $month[zeroise($arcresult->month,2)], $arcresult->year);
to
$text = sprintf("%s %d", $month_abbrev[$month[zeroise($arcresult->month,2)]], $arcresult->year);
This is all assuming you use version 1.2.2. If you use an earlier version, the changes, I think, are the same, but the line numbers may differ.
EDIT: That will get you your month abbreviations. To get the number of posts, all you have to do is use
<?php wp_get_archives('type=monthly&show_post_count=1'); ?>
in your index.php.