According to the raws, 1 seed is produced when you: "make sheet from plant" and "mash plant into slurry", though the slurry to paper does not yield another seed (unsurprisingly).
[REACTION:MAKE_SHEET_FROM_PLANT]
[NAME:make sheet from plant]
[BUILDING:FARMER:NONE]
[REAGENT:plant:1:PLANT:NONE:NONE:NONE]
[REACTION_CLASS:PAPER_PLANT]
[UNROTTEN]
[PRODUCT:100:1:SHEET:NONE:GET_MATERIAL_FROM_REAGENT:plant:NONE]
[PRODUCT_PRESSED]
[PRODUCT:100:1:SEEDS:NONE:GET_MATERIAL_FROM_REAGENT:plant:SEED_MAT]
[SKILL:PAPERMAKING]
[REACTION:MAKE_SLURRY_FROM_PLANT]
[NAME:mash plant into slurry]
[BUILDING:QUERN:NONE]
[BUILDING:MILLSTONE:NONE]
[REAGENT:plant:1:PLANT:NONE:NONE:NONE]
[HAS_MATERIAL_REACTION_PRODUCT:PRESS_PAPER_MAT]
[UNROTTEN]
[PRODUCT:100:1:GLOB:NONE:GET_MATERIAL_FROM_REAGENT:plant:PRESS_PAPER_MAT]
[PRODUCT_PASTE]
[PRODUCT:100:1:SEEDS:NONE:GET_MATERIAL_FROM_REAGENT:plant:SEED_MAT]
[SKILL:PAPERMAKING]
[REACTION:PRESS_PLANT_PAPER]
[NAME:press plant slurry into paper sheet]
[BUILDING:SCREW_PRESS:NONE]
[REAGENT:paste:150:GLOB:NONE:NONE:NONE]
[REACTION_CLASS:PAPER_SLURRY]
[NOT_PRESSED]
[PRODUCT:100:1:SHEET:NONE:GET_MATERIAL_FROM_REAGENT:paste:NONE]
[PRODUCT_PRESSED]
[SKILL:PAPERMAKING]
I think the sheet from plant is the papyrus, well the mash plant to slurry is pressing of cloth plants (pigtail, hemp, etc). It's possible they grabbed the seed right before you saw it in the workshop, so you didn't see it (as some dwarf was hauling it).
Edit: Also, I think paper wise papyrus is the easiest to make paper from, but you have to have it on site (or have it traded to your fort that's on a site that can plant it, but it's not local for some reason). If you're site can't grow papyrus, the plants->slush->paper is just as renewable, but has more steps (and can be done underground), and then parchment is nice if you have a surplus of the needed boulders and use hunting/breeding as a food source (as it uses skin).
They are kinda akin to fuel, in that you have one that is not found everywhere (papyrus/magma), one that can be found pretty much everywhere (slurry/charcoal), and one that you have to dig up and process a bit (parchment/coke). (not an exact match, just a bit of a parallel).