Generally, you use fler for countable nouns, and mer for uncountable. So it's e.g. mer mjölk ("more milk") because you can't really say "I have four milks", but it's fler stolar because you'd say "I have four chairs".
It's the same difference as with "fewer" and "less" in English (fewer chairs, less milk). Funny that Swedish uses this differentiation when talking about greater amounts/numbers, while English uses it for smaller amounts/numbers.
Swedish has also ''mindre'' and ''färre'', which translate to ''less'' and ''fewer'' respectively, if I'm not mistaken.