I've heard a while ago that there will be a limited numer of bitcoins available and that at some point all bitcoins will be mined.
is that true ?
Yes.
how many bitcoins are still out there to be mined and by what year will all be mined ?
The total amount of bitcoin is hardcoded to be 21 million BTC at max. The mining rewards half after a certain amount of time, which will result in almost 21 million btc sometime in the next century (year ~2140)