Niko can date five different girls throughout the game.
Michelle and Kate are girls that he meets while the story is in process, and the other three he meets through contacting them on the internet.
Niko meets his first girlfriend, Michelle, during the mission Three's a Crowd.
Do not call her when she’s not awake or her fondness of Niko will go down.
Also once you arrange a date with her do not take over an hour in the game’s time to pick her up or her fondness will also go down.
After requesting a date, you'll have to wait awhile (a week or more) for a reply. Reply to the e-mail to trigger the appointment, and be sure to show up on time.
After the initial date, you can then collect the girl's phone number so you can call and set up additional dates.