Learn Russian: У тебя есть два компьютера.

This is the golden rule for numbers 2, 3, and 4 in Russian! After these numbers, you do not use the plural form. Instead, the noun must be in the Genitive singular form. Since компьютер is a masculine word ending in a consonant, we add -а to make it Genitive singular: компьютера.

Why does the sentence use два instead of две for the number two?

In Russian, the number "two" changes depending on the gender of the noun it counts. We use два for masculine and neuter nouns, and две for feminine nouns. Because компьютер is a masculine noun, we must use два.

Since I "have" the computers, shouldn't the computers be the direct object?

In English, yes! But remember that the literal Russian translation is "At you, two computers exist." This means "two computers" is actually the grammatical subject of the sentence, doing the "existing." The noun компьютера only changes its ending because it is following the number два, not because it is having anything done to it.

A quick refresher: how does У тебя есть translate to "You have"?

Russian doesn't normally use a direct verb for "to have". Instead, it uses a prepositional phrase that literally translates to "At you there is". У means "at" or "by", тебя is the Genitive form of "you", and есть means "is" or "exists".