Learn Arabic (Modern Standard): كم وترا في الجيتار؟

In Arabic, the question word كم (how many) must always be followed by a singular noun. Even though you are asking about multiple things, the rule is to literally ask "How many string?".

Why does the word وترا end with an extra Alif?

The Alif is there to hold the "tanwin fatha" (the "-an" sound ending), making it wataran. In Arabic grammar, the singular noun immediately following كم must be in the accusative case, which is marked by this ending.

Where is the verb "are" in this Arabic sentence?

There is no verb! Arabic does not use a present tense "to be" verb like am, is, or are. The sentence literally translates to "How many string in the guitar?" and the "are" is naturally understood.

How does Arabic handle foreign words like الجيتار?

الجيتار (al-jītār) is a loanword, but Arabic treats it just like a regular native noun. You simply attach the standard definite article ال (the) to the beginning and apply normal preposition rules, as seen here after في (in).