From @FrozenStorm and @Bryce_The_Rice’s game:
How about:
“Whenever you play a unit from your hand you may exhaust it. If you do, draw a card for each of your Flagstone Garrisons”
which repurposes the wording from Galina Glimmer.
Other option is the wording from Gigadon, which would be “… for each Flagstone Garrison you have”
I kind of prefer “…for each Flagstone Garrison you control” as it seems like the least ambiguous way to phrase the condition.
My only question is, would these wordings be guaranteed to only trigger once for each exhausted unit? @charnel_mouse you seemed confident interpreting the “if you do” clause - thoughts?