Learn German: Der Bus hatte fünfzig Plätze.

Yes! Just like English adds '-ty' to make fifty or sixty, German adds -zig to the base number. So fünf (five) becomes fünfzig (fifty). Note that the -ig at the end is usually pronounced like a soft 'ich' sound.

Is hatte the past tense of haben?

Exactly. hatte means 'had'. At this level, you mostly use a two-part verb to talk about the past (like 'have played'), but haben is a common exception. Germans usually prefer its one-word past form hatte, just like we say 'had' in English!

I learned that Platz means 'square' or 'place'. Why does it mean 'seats' here?

The word der Platz can mean a city square or a general place, but on a bus, train, or at an event, it means 'seat'. To make it plural (seats), German adds an -e at the end and puts an umlaut over the a, making it die Plätze.

How do I pronounce the 'z' in fünfzig and Plätze?

In German, the letter 'z' is always pronounced like 'ts', similar to the end of the English word 'cats'. So Plätze starts with a 'plets' sound and fünfzig has a 'ts' sound right in the middle.