How do I find the number of messages I've sent to someone on facebook?

[Kahdeem]

[M]

If you click into a message and scroll up really quickly, it will tell you the total amount of messages in that message thread.

Also you can always click into "see all" under the Messages icon and search for the person which shows you how many separate message threads you have with that person.

[1lastBr3ath]

I don't know exactly how on PC, but I can tell you how on mobile.
You can use any mobile browser like UC Browser (not Facebook app). Here's how:
- Open any particular 'Message'
- Select "See Older Messages" and copy its link [Do Not Click]
The link will look like: https://m.facebook.com/messages/read/?de...1&refid=12

Notice 'start=6811'?
It's the total number of messages (Conversation) in previous pages. Hence,
Total no. of Messages=6811 + 5 = 6816
There are 5 number of messages in a page, by default.
In this way, you can calculate number of messages you've sent ( + recieved) with someone. I don't think you can find number of messages only you've sent.
Hope it helps.