For me, if the emojis are at the end of the post, it doesn't matter because presumably I've already gotten the message of your post so I can scroll on. But if they're at the beginning or in the middle, probably 3 is my limit. I think the worse thing is the ascii art, not necessarily the emojis, because while screen readers are getting better at dealing with emojis, they don't know how to deal with ascii art at all and so it's just a bunch of font information and single-character reading. Hope this helps.