You do not need any portal to project to the astral plane.
If you are talking about a spell for astral projection, then I have probably never heard about such thing including the word 'portal'.
There have been spells designed and potions brewed to help induce astral projection but the actual splitting itself is done by willful or subconscious splitting between the astral and physical bodies. That's what you need to know.
If you're talking about seeing portals while astral projecting, there's no problem to do that.
The astral environment is thought and feeling sensitive. Your thoughts will present themselves before your eyes so it's wise to pay attention to what you have in your mind and not let your thoughts run wild.As I said, it works with thoughts, positive and negative energy, etc. Same thing happens with many other things such as: fear, bad thoughts and negative energy.
While astral projecting we see what we have in our minds and subconscious.
To say this straight to the point, thoughts are manifested in the astral realm while astral projecting. Meaning: if your thoughts focus or portal while attempting to astral project, that's what you'll see due to the manifestation.
If by saying portals you're talking about something associated with fantasy... Give it up.