Dear Keshav, 

At the time of filling the application form, you will be able to give the preference for all the participating NLUs, i.e. 21, since 21 NLUs are participating in CLAT UG 2019. 

Coming to how you should decide it, you can take help of the rank of law colleges given in the webpage mentioned below. There are varied parameters of marking these, such as research base, infrastructure, placements, et.al. 

Furthermore, you can also prioritize for yourself as to which parameter is more important for you. For instance, if you are aiming towards entering the practice right after, you can look at the placement records of the law colleges in consideration, if you want to develop a sense of doing research and aiming towards academia, you can choose a college with advanced degree law programmes, etc. 

https://law.careers360.com/colleges/ranking

Hope it Clears! 

All the best!