Creating 7 for 7 different "from" addresses should work. Each would be a "Signature 1" for each address.
However, I also see an "All Signatures" entry in the Signatures box. Maybe it is for generic content for all boxes? I don't know. Put something in that one as a test and open a few new emails from different "From" email addresses and see if they show the test content you insert. If not, yes, create a Signature 1 for each email address.
Usually multiple email addresses serve different purposes, such as one is work and one is personal. Presumably, the assumption is that you would want different signature information in such email. If that's not true for you, perfect a signature in one, then select it, copy, and paste the same into the others as their own "Signature 1"
Clicking the question mark icon in the lower right says this about "All Signatures": When All Signatures is selected, you can add an existing signature to another account by dragging the signature from the middle column to the account in the left column. You can use the same signature in more than one account. So instead of copy & paste, that looks like the faster way to copy a signature to other email address in your list of 7. Presumably, this would make it so that if you tweak the address in one place, the change would take effect for all of the from email you send (vs. having to make the same tweak 7 times).
You might want to click that same question mark icon and read the help instructions that open. They seem to explain much of this quite well.
