A green dot in the corner of the StreetPass Mii Plaza icon in the HOME menu of your Nintendo 3DS system indicates you have received a Mii character through StreetPass. If your Nintendo 3DS system is closed, but powered on, a green light on the top right of the closed system will indicate the same.

You can meet a maximum of 10 people per session in StreetPass Mii Plaza. Once you've received 10 Mii characters, go to StreetPass Mii Plaza and welcome them. After you've done this, you can receive new Mii characters again. Note that until you've welcomed these 10 Mii characters, you will not exchange your Mii character with any other system you may encounter.

You can encounter another user and their Mii character an unlimited amount of times, but you'll need to wait 4-8 hours between each encounter before you can meet the same person again.