Learn Spanish (Latin America): Seiscientos turistas visitan la ciudad hoy. - Six hundred tourists visit the city today.

It is simply made by combining seis (six) and cientos (hundreds). Many hundreds in Spanish are formed this exact same way, like doscientos (two hundred) or trescientos (three hundred).

Since turistas ends in an 'a', shouldn't the number be seiscientas?

The word turista is the same for both men and women (el turista or la turista). When referring to a general or mixed group of people, Spanish defaults to masculine grammar. Because the hundreds from 200 to 999 must match the gender of the noun they describe, we use the masculine seiscientos here.

How do we know ciudad (city) is feminine and needs la?

While most feminine words end in the letter 'a', nouns that end in -dad are almost always feminine in Spanish. This is a very handy pattern to remember!

Why does the verb visitar (to visit) end in -an here?

The verb is visitan because the subject is the tourists, which is a 'they' (ellos). For regular verbs ending in -ar, the 'they' form always ends in -an.