Ooooo boy. That's a loaded question! I bet you will get as many answers as members here! Lol... Honestly- if you have a good sealed fermenter, you can trust the first purge. Here is my process. YMMV, and I think a lot of ways will work.
I have a Spike cf5 with temp control coil and glycol chiller.
#1: After I fill it with wort, I seal it up. I then immediately purge with c02. At least 3 times. (Note- If i had a pump, I could pump wort into it like a lot people here do.)
#2: I add about 5psi C02 and set the temp coil to the temp I want. (my wort comes in about 75-80 or so, It only takes about 15 minutes to reach desired temp- usually 68 or so)
#3: When I hit temp, I use the carb stone and blast it with Oxygen.
#4: I then open the 1.5" tri-clamp pressure gauge and dump in my yeast.
(Note- sometimes I will add yeast, then aerate, Again- many discussions to be had. )
#5- I will then purge again with c02 once.
#6- I will attache a blow-off tube and let the fermentation begin! Technically, there is a bit Oxygen in the head-space. I once decided to wait 12 hours for fermentation to start before adding the blow-off tube (so I could purge with c02 and ensure no oxygen). The problem with this is... I barely made it in time to add a blow-off tube. It is just not worth it to me.
Sorry, this is rambling. May I suggest you read some on this thread- it is about the Spike CF series, but a lot of handy info!
https://www.homebrewtalk.com/threads/spike-conical-observations-and-best-practices.645440/