There is a hard cap on the bonus damage from intelligence is at 99 intelligence. At this point the game no longer lets you put points into the stat (99 is the hard cap for all stats). Intelligence never stops increasing your magic damage so you will see an increase in damage as you increase intelligence.
However, as you increase the intelligence stat higher and higher you will get diminishing returns. According to this page on the Dark Souls 2 wiki the amount added to your magic bonus by increasing intelligence is different depending on what value the intelligence stat is at. So when your intelligence is between 31 and 40 each point you put into intelligence will increase the magic bonus by 30. Between 41 and 50 the increase is 15. Between 51 and 60 the increase is 7. At each of these steps the bonus has halved. The increase stays around 8 until intelligence is 99 (in the range of 71 to 80 it hits 15 again but for all other ranges it's 8).
This trend of getting less the higher you go is why many guides say that the soft cap for intelligence is 40 or 50. Past these values you're getting very little increase to your damage so it is worth it to increase other stats (as it costs the same to level up no matter which stat you're increasing).