おばあちゃんの家でたくさんペットを飼ってるって英語でなんて言うの？

Many native speakers from England use the title "Nan" when speaking about their Grandma. This is because many people feel that "Grandma" is a title for old women only and they do not want to be thought of as old because they may only be between the ages of 40 - 50 for example.

If you think that the number of pets your Nan has is a lot more than what is usually normal you can say "so many," emphasising the "so."

The term "looks after" is another common way to "cares for"or "takes care of"but can also be used when only doing so for a short time for perhaps a friend or family member.

If someone has a lot of pets, it's can be useful to explain that they have a love or passions for caring for animals to explain why they have pets.
Using similes can be useful when explaining if someone likes pets. By using the simile of a zoo it indicates that a lot of animals live somewhere.

There is a name for people who have lots of pets, they are called "Cynophilists"

A lot of elderly people do not want to be alone, so, they surround themselves with animals. It keeps them busy and they don't feel lonely.

"My Grandma Owns many pets" this sentence refers to owning or possessing pets. to own is to keep/have/raise.
"My Grandma" can also be referred to as Grandmother,Gran,Granny,Nan,Nanny etc..

to say that your grandmother has many pets, one can simply say "my grandma has lots of pets." or "there are many pets at my grandmother's house." we use the words "many" and or "a lot" to express that there are more than just one, meaning many.

My grandma is a pet lover. - This sentence expresses the love your grandma has for pets. Someone who is a pet lover is someone who loves having pets.

My grandma lives with all her pets. - In this sentence, the word "all" expresses the idea that your grandma has many pets.

Your grandmother has a lot of pets in her house. You would like to explain this situation. Above are a couple of suggested statements which you may find appropriate.

If you would like to explain that your gran has many pets at home, you can use the sentences above.
Some people have many different pets at home, they have a lot of patience and love for animals.

"My gran's house is full of pets, she owns all of them!"

Own: to be the keeper of something

Grandmothers are old and lonely and often rely on household pets to keep them company. They take care of these animals and raise them as if they were her children. She feeds them and looks after them.

My grandma's house is like a zoo!'
This is an informal way to saying that your grandma has a lot of pets. If we were to visit the zoo, we would see a lot of animals and if we visit your grandma's house, we would also see many animals! Therefore, we can compare it to a zoo!