"Do you have stamps?"
Translation:Hast du Briefmarken?
If you can read German, then have a look at the Wikipedia article Fugenlaut: https://de.wikipedia.org/wiki/Fugenlaut
Essentially, though: it's probably best just to learn compound words as single items rather than trying to derive some rule as to whether a sound gets added between the two and if so, which one (-e-, -n-, -s-, something else).
Sometimes, there are even multiple possibilities with different meanings: a Gasthaus is not the same thing as a Gästehaus, for example.