It's not quite that easy, unfortunately. The ending depends on whether the last 'e' before the 'r' is stressed (keep the 'e'), and whether it follows a double consonant (drop the 'e').
Indefinite singular m/f nouns ending in -er usually get an extra -e added, but there are some other inflection patterns as well. "Mester" has three different patterns to choose from:
en mester, flere mestere
en mester, flere mestrer
en mester, flere mestre
Indefinite singular neuter nouns ending in -er tend to stay the same in the plural, often with the option to invert the ending to -re:
et kateter, flere kateter
et kateter, flere katetre
There are also some cases where the plural gets an -ere ending, but it's less common than for m/f nouns. Here is an overview (in Norwegian) of the different patterns.