Runestones and writs are one per spell. This is designed to be slightly restrictive - more in line with the low number of spells memorised by magic users and clerics in b/x d&d. At some point your character might well own several more runestones than they can carry, this means they will need to choose which ones to take with them (similar to memorising), or come up with ways to carry more, e.g. hiring a porter.
Feel free to join us on the FORGE discord where we've talked about this, as well as many other topics. There are also options for increasing carry capacity if RAW feels too restrictive - everyone has different preferences!