Learn Russian: У дедушки есть старый друг.

Russian doesn't normally use a verb like "to have." Instead, it expresses possession using the construction У + [owner] + есть + [possessed object]. Literally, this sentence translates to "At grandfather there is an old friend."

дедушка is a masculine word. Why does it end in -и in this sentence?

After the word У, the owner must be in the Genitive case. Even though дедушка (grandfather) is masculine, it ends in -а, so its ending changes just like a feminine noun. Normally, -а changes to -ы in the Genitive case, but because of a strict Russian spelling rule (you cannot write -ы after the letter к), it changes to -и instead, becoming дедушки.

Why isn't старый друг in the Accusative case, since it's the object he has?

In English, "friend" is the object of the verb "has." But in the Russian У ... есть construction, the possessed thing is actually the grammatical subject of the sentence (literally: "an old friend is at grandfather"). Because of this, старый друг stays in the basic Nominative case.

Would the word есть change if grandfather had multiple friends?

No. In this basic possession construction, есть stays exactly the same whether he has one friend or many friends. You would simply change the noun phrase at the end.