That starts to get complex as the number also has to take into account pagination (e.g. 100 signatures to a page).
I haven’t tested thoroughly, but this works by the look of it.
in /wp-content/plugins/speakout/includes/class.signaturelist.php
Line 40 under the line $current_signature_number = $total - $start;
add an extra line $current_signature_number = $start + 1;
line 163 & line 239 change $current_signature_number --;
to $current_signature_number ++;
BUT, make a backup of the original file before you make any changes and also once you have done it and it is working, make a copy of the file you just edited so that if an update over-writes them, you can easily redo it.