Here is the experience my husband and I had (including the advice from the immigration lawyer) with regards to letters...
The letters are better written in professional language. There's no set format from the government, but the lawyer recommended that each letter should be unique/personally written (i.e., do not just copy-paste letters and have each friend/family member sign off on a copy). Our lawyer recommended to include all of the following:
-date of writing/signing
-be addressed to IRCC (note that while some of our letters had a letter head including the IRCC address, others simply wrote "Dear IRCC Officer:"
-a brief introduction of the writer (name, professional background)
-how the letter writer knows you and your spouse, including specific details of how you met, when you met, how long you've known each other, how often you see each other/keep in touch
-times the letter writer has seen you and your spouse together, and times the writer spent with you and your spouse together
-affirmation of your genuine love for each other and the specific aspects of how they see that each of you are upstanding members of society and how you make a good couple
-if you are expecting the IRCC officer to consider your relationship to have "red flags" (e.g., cultural background difference, significant age gap, education level difference, short period of courtship, etc.), and your letter writers are comfortable addressing these things, have them write about how those "red flags" don't hinder you as a couple
-the contact information of the letter writer (phone number at a bare minimum, preferably also e-mail) so that the officer may (if they so choose) contact them to verify the person who wrote this letter is genuine
-the letter may be typed or hand-written (as long as the writing is easy to read), but ideally the letter should be signed by hand. Our lawyer stated that in the event that someone is unable to hand-sign the letter for any reason (one of our letter-writers could not provide a hand-written signature), they must provide their contact information in the letter so that the IRCC officer reviewing the case can contact them to verify the authenticity of the letter if needed
Our immigration lawyer also consulted with a senior criminal lawyer (since my husband has 4 criminal charges on his record), and with regards to the letters, the criminal lawyer recommended including a photocopy of the letter writer's photo ID so that the IRCC officer could have a face to the writer and feel more personal. Our immigration lawyer said that it is not necessary to do this, though you can. (We did not end up including photocopies of the writers' photo IDs.)
In terms of the quantity of letters, our immigration lawyer wouldn't give us a straight up answer about how many is recommended... when I said I could easily get 50 letters from my co-workers and friends and family however, the lawyer immediately said that 50 was far too many letters, and to restrict it to close friends and family so as to avoid overwhelming the IRCC officer.
In the end we only included 3 letters (one from my brother, my dad, and my husband's mom), all from close family members who attended our very small wedding and were shown in our photos. However, I'm sure that you could acceptably include more than that, especially if you do not have much in other types of proof (e.g., pictures, call/chat logs, shared bills/accounts, etc.).
The letters we submitted ended up being 1-2 pages long. There isn't a specified length maximum or minimum, but as long as your writers are clear, concise, and professional, it should be fine.
All of our letters were written in English and did not require translation or notarization. I am uncertain about the requirements for foreign language letters.
Hope that helps!