One is using 5 or 10 % Pd on carbon to conduct hydrogenation of a number of functional groups in substances, then other factors are the solvent; the hydrogen pressure one provides to the Parr apparatus (and conduct the shaking in a constant way). One can calculate the amount but Pd/C is a catalyst which means that after reaction it will be present in almost quantitative way. I would advise to check the literature and take into account the amount of Pd/C which is reasonable. At the limit if you would use equal millimoles Pd/C and substrate, your reaction might not proceed quicker. In making 100 substances during my Ph.D I have always used say 50-100 mg for 20-25 g of substrate (I NEVER calculated millimoles). One reaction I recall is the conversion of 4-nitro-anthranilic acid to 4-amino-anthranilic acid with Pd/C in ethanol overnight at 3 bar hydrogen overnight.

I would say the best approach is to check the mechanism of the reaction, what is the role of the Pd catalyst in said reaction?

For example, in a Sonogashira coupling, the Pd cycle starts with the oxidative addition of the halide, so in my calculations of how much should I add, I take into consideration the mmols of halide I'm adding to the reaction.

For hydrogenations I would just follow what Johan De meester said, up to 10% of catalyst is a reasonable amount.

As you said for hydrogenation, up to 10% of the catalyst is a reasonable amount. Can you also suggest how much % catalyst should be used in the case of Heck coupling or Suzuki coupling?

Can you please refer any article related to this?

I would suggest you to start with something around 0.1 mol-% of Pd catalyst, and start increasing to see if the yields get better.

You can probably find many reviews on the topic or even a couple of articles dealing with something similar to what you want to do.

For a general overview of both reactions check:

https://www.organic-chemistry.org/namedreactions/heck-reaction.shtm

https://www.organic-chemistry.org/namedreactions/suzuki-coupling.shtm