How do you count how many DPO you are?? Confused!!๐Ÿ˜ฉ๐Ÿ˜ž

Hailey โ€ข i have a year old son, who is my whole world! Im currently trying to get pregnant. I would love to give my son, a little brother or sister!โ˜บ
So I thought I was 2DPO but I'm not for sure.... Glow is saying I ovulated on the 3rd. Ovia is saying I ovulated on the 4th and said to take a HPT in 10 days. And my <a href="https://play.google.com/store/apps/details?id=com.glow.android">ovulation tracker</a> is saying I ovulated on the 5th..... I'm so confused!! My last period was Nov. 18th. My period this month isn't due until Dec. 18th. So can someone plz help me?! Idk when to test or how many DPO I am..... Ugh! ๐Ÿ˜ฉ๐Ÿ˜ž๐Ÿ˜ข Plz help!!!!!!!!!