There are many ways you could do this, but that depends very much what distances you want to do each day. On the
https://godesalco.com/ planner, the distance from Lisbon to Porto is 384 km, but that is through Alpriate, which is no longer on the route. Over 16 days, that would be an average of 24 km/day. To average that might require days well over that to compensate for the days where you will inevitably walk less for some reason.
Even when I set a daily target of 20 km/day on this route, my longest day was about 29 km just because of the lay of the land and where towns are located. My shortest day was about 16 km for similar reasons. You might find a pattern where you can do it in 16 days with some days around or just over 30 km. I couldn't have done that.
The other complexity for me was leaving Lisbon. Eventually I decided to walk to the station at Povoa, return to Lisbon, and go out by train again the next day. I did something similar on the leg from Azambuja to Santatrem, catching a bus and train from Porto de Muge then returning to Muge (on the other side of the Rio Tejo from Porto de Muje) by bus the next day and walking from there.