Assalamualaikum Ustadh, If someone intentionally didn't pray all there fard Salah and only prayed 2 or 3 everyday or even just once a week for many weeks and they don't know many Salah they missed, what do they do?

There are saying for the people of ilm. Some said u have to make them up until you think that most probably you made them up. The other saying is that u don't make them up, repent to Allah, and increase in voluntary prayers.