"Supernetting" is a non-standard term for when the network is *bigger* than you would expect.
"Subnetting" is really a vestigial term for the most part. i.e. you don't have a subnet, you have a /23 network, period. UNLESS you now subdivide that network space for internal use, that is.
Your /23 mask means the first 23 bits of the 32 bit address are the network, the remaining bits are host.
Your specific range in the example give is
209.124.34.0 - Network
209.124.34.1 - First valid Host address
209.124.35.254 - Last valid host address
209.124.35.255 - Broadcast address
Total of 510 valid host IPs.
( In the new enlightened age of CIDR this should be the end of it. However if you have any legacy equipment around which is trying to think class-full addressing, you will probably find it won't like to talk to 209.124.34.255 or 209.124.35.0 as valid host ips, even though they *are*, so it might be best to avoid using them if you can. )
Now if you wish to sub-divide that into subnets - well - the question is, how big do you want them to be.
You can have two /24 networks, each with 254 hosts
209.124.34.0/24
209.124.35.0/24
You can have four /25 networks, each with 126 hosts
209.124.34.0/22
209.124.34.128/22
209.124.35.0/22
209.124.35.128
You can have up to eight /26 networks of 62 hosts each
etc.
But you don't have to use equial size subnets, so you can have
one /24 at 209.124.34.0/24
one /25 at 209.124.35.0/25
two /26 at 209.124.35.128/26 and 209.124.35.192/26
just for one example.
So the answer to "how many subnets can I have?" depends on how big you want them to be!
If you can't do it in your head (most can't!) then I commend
www.subnet-calculator.com/cidr.php
