Mostly "pengar" means money, and is then always used in plural, and as an uncountable noun (e.g. "mycket pengar")
However, "en peng/pengar" can sometimes be used instead of "ett mynt/mynt" to mean a coin. Then it is a countable noun (e.g. "mång pengar"), however I think that sounds like something mostly used when talking to a child.
Also "peng" can sometimes be used, usually as a suffix, for a sum of money given to someone. For example a weekly allowance is "veckopeng". Then it is always singular (e.g. "stor peng", or indirect "mycket i/som veckopeng")