What are you trying to achieve by using web gardens? If it is increased performance, then definitely do NOT do this. Web gardens, while advertised by Microsoft documentation and books as increasing performance, typically tend to decrease it - especially if the bottleneck in your application is the cpu.
What they are used for is if you are having reliability problems (your application pools keep crashing) - the web garden can make your application more reliable, but you will probably take a performance penalty for that.
Here is a great blog post that walks through whether you should use web gardens or not:
Also, here is Microsoft's documentation on when it could make sense to use a web garden for performance reasons - but maybe take it with a grain of salt: