can I use 20W adapter for iPad (5th generation)?

Yes. The 20W USB-C Power Adapter is perfectly usable with your iPad5 - although your iPad will not charge significantly faster. The iPad5 has a maximum supported charging rate of about 12W (2.4A @5V).

You iPad will only draw the power that it needs - up to the maximum power that the iPad is designed to charge its battery. The iPad will automatically regulate battery charging, reducing the current (mA) that it draws from the Power Adapter as full charge is reached.

Yes: the iPad will just take the power it needs.