Michael explained everything very well, just a few added comments.

If the bus is short and not too fast, you can get by with a lot, the 485 driver can drive 2*120 plus 32*12k in parrallel, the only problem with mistermination is reflections which must "die" within less than 1/2 bit time.

Another potential problem is the available power supply current on the board the 485 transciever is located on. Once I has 485 problems when adding loads to a bus and upon query, the designer replied: "it is only intended to have one device connected". It turned out that a hefty capacitor (tantalum and ceramic in parrallel) across the 485 driver on the offending board solved the problem.

Erik

PS: you will have many "strange" problems if you do not do some hefty decoupling of the power to the 485 driver.