I don't, they're all dead.
English ones Grandma and Grandad, Welsh ones Nain and Taid.
(When they were alive) I called them on my mum's side gran and grandpa; on my dad's grandad (the grandma was already dead when I was born so I don't know what I would have called her).
3 died before I was born. The last remaining one (my maternal grandmother) I call nana
Mum's mum: Grandma
Mum's dad: Grandpa
Dad's mum: Nannie
Dad's dad: Granddad
Grandma's mum: Nana
Grandma's dad: Granddad Jack
Grandpa's mum: Gran
Grandpa's dad: Granddad ("Great Granddad" always sounded too formal).
Never met my dad's grandparents, I assume they're dead seeing as my dad's parents are in their 80s. Then again, my great (great?) aunt lived to be 104.
Nan and Grandad - basic *****es.
Nana, Nani, Bibi, baba (depending on mums side or dads side)
Never met mine, never met my parents either. I have 16 grandchildren. Older ones call me Nan, some call me Nanny, some Grandma, and the babies just say Namnam.
Grandma and Grandad. Mum's biologcial dad we call by his real name. (because that's what mum calls him and it would be weird to now call him Grandad)
Can't remember what we called dad's mum. Dad's stepdad we called by his real name, because that's what dad called him by.
Dad's parents - Grandma and Grandad.*
Mum's mum - Granny. Never knew my grandfather on my mum's side. *