How do you know how many power ups your trainer level will allow?

It's your level + 1.5.

So if you're level 36 then you can power up a mon to 37.5.

Ok so the max power up is your lvl +1.5 for example 35 = 36.5 also each power up is .5 so every lvl is 2 power ups.

For 38 the Pokemon max level is 39 through.

Besides to know the +1.5lvl thing, i really like to use GoIV app to get a clue about how many candys and stardust it will cost to level a specific Level.
Since the App is just an overlay and uses no Login or anything, this should be as legal and fair as any other IV/dust calculator.

If you want your Rhydon to be level 35 you can do the math using an IV Calculator, it will tell you its current level and this way you can know how many power ups you need to make it go where you want.

For example if your Rhydon is level 33 and you want it to go to level 35 it will take you 2 power ups per level, a total of 4 power ups.

Another way to determine where you are is looking at the tables on this site, so a mon that is level 35 will cost 8000 dust per power up.

There is also the CP Calculator under the IV Calculator on this site - calculates exactly the amount of candy/dust/kms/etc needed to get a mon to a certain level. I use it all the time!

Download pokegenie, which is amazing! Strongly recommend.