Just two bolts for the front of passenger seat and on for the rear of said seat....I have a two piece on my IS. Sorry don't remember sizes but can be bought at any hardware store.
Thank for the reply.

So you mean the rear seat is bolted to the rear fender and the front seat is hook to the front of the pillion seat?
No, the seats are independent of each other on the Standard/Tourer.
The rear of the drivers seat hooks to the metal slot thingy on the rear fender and the key lock at the front. A S/T OEM drivers seat will be longer due to the larger I/S tank, (or smaller S/T tank) but with some extra force, it should fit.
The pillion seat bolts with 3 bolts (assuming your pillion has the brackets) which you will have to source from a hardware store or parts seller.
#20 & 21.
https://www.partzilla.com/catalog/honda/motorcycle/1998/gl1500c-a-valkyrie/seat