Change Gmail Conversations on Page
Be default Gmail shows 50 conversations in one page, however its possible to increase or decrease this number.
These are email conversations, so if you have 10 emails back and forth with same subject, Gmail with group them in a thread and will count it as one conversation.
Gmail Strikethrough Shortcut
Above method is a work around, but you will not want to do this all the time, as back and forth between Google Docs and Gmail might be painful to achieve this small task of striking through your message.
You can create a Canned response in Gmail with your strikethrough text one time, and then whenever you need to strikethrough, just insert your canned response, edit it on the fly, and you are all set:).