You can have more than 32 devices on a 485 network. 485 allows 32 "units load" on a bus - most transceivers these days are 1/4 load devices (or less) and then you can add repeaters to connect two 485 buses. The Modbus specification allows 247 slaves on one serial Modbus network. The limitation is if the devices allow you to address them up that high - some devices only allow you to address to 32 or 64 or 128, etc.
You will probably not meet your 1 second poll time requirement at 9600bps, even with 16 separate networks. This of course depends on how much data you are requesting from each slave. If you are reading a small number of consecutive coils, then you could probably do it, but if you are reading a handful of registers, then you will be too slow.
Here is some math for you on reading a single register using Modbus RTU:
- Start delay - 39 bit minimum
- Request of single register - 64 bit payload becomes 88 bits on the wire
- Stop delay - 39 bits minimum - turn around on the slave could be much longer
- Response - 56 bit payload - 77 bits on the wire
Total 243 bits "wire time" minimum to read one register. So to read a single register from every device @ 9600bps with a one second poll time you can read from 39 devices max (9600/243). You probably want to read from more than one register too, each additional consecutive register will add 22 bits to the response. If they are not consecutive, then you have to make an entire new request again.
Are your slaves able to communicate faster than 9600bps? How many registers/coils are you reading/writing? Are you needing to read from all slaves all the time?
What I mean here is if you have one chart calling the modbus subroutines, then you can only be communicating on one serial bus at a time. The subroutines will wait for the slave to respond (or timeout) before they return to you. During this time, your chart is doing nothing, just waiting. You will need separate charts for each serial network if you want to communicate simultaneously. If you are running 16 serial networks, you will need 16 charts running - which I believe is the max on an R2, so that is a concern.