Why is leibh used instead of leat?
leibh is plural/formal; leat is singular.
Don't you mean Tapadh leat?
Shouldn't leibh only be used when it is more than 3 people, due to how plurals work in Gaelic?
How are we supposed to know to use the informal or formal when using names? As far as I know Eilidh and Anna are my acquaintances.
Leibh is also used as the plural form. Since you're thanking Eilidh and Anna, you need to use leibh rather than leat.
I was also wondering why leibh was used instead of leat. I understood leibh to be used to address someone older or as mark of respect. Did not know it also plural/singular form.
Taing dhuit should be accepted