The general question you ask is like asking how much gas a car needs. Different cars need different amounts, and so it is with air for amplifiers. I have a couple that run quite cool. My ODL has massive heat sinks and never gets very warm, and it does not have any air holes (so the circuitry inside never gets dust and dirt on it either). My Crown K2 gets warm, but never hot, and it is also sealed, so no dust or dirt gets inside.
Now, looking at your amplifier, one can see that almost the entire top of the case is a grate, and so we can guess that they expect a lot of heat to be coming out of there. I recommend several inches above it. But one cannot say precisely how many inches, both because I am not familiar with that specific amplifier, and also because how much space is needed will also depend on the room temperature, as well as how much heat is generated by other devices close by, and also it will depend on how hard you push the amplifier.
Just remember, there is no such thing as too much space over it and empty space around it; there is only such a thing as too little.
I also do not recommend putting an amplifier on a piece of cloth, especially if it has vents on the bottom. Cloth insulates (and can block vents if present), and you don't want to keep the heat in, you want the opposite.
If you have the manual for it, it will likely give you advice on this subject. I recommend following such advice, and never giving an amplifier less space than the manufacturer recommends.
