Hi, Ray,
The nice thing about the forum-produced guide to the Lisbon-Porto section
https://www.caminodesantiago.me/community/resources/lisbon-to-porto-2015-version.298/
is that I have tried to include descriptions from 2014 pilgrims of places where they had trouble finding the arrows and how they found their way. No one, by the way, reported any serious problems. You are never out in the wilderness and far from civilization, so "getting lost" usually means you walk a km or two out of your way. But there should be absolutely no worry. I walked alone in 2009 and once or twice I got turned around but I always was able to get where I was going. And since then I know there have been tremendous improvements in signage and arrows.
For added security, though, you could carry with you the SOS pilgrim phone number. It's staffed by members of Via Lusitana, they speak English and other languages, and you can call them any time day or night. Particularly with Lisbon-Porto, they know the route by heart and can help you out in a pinch. The number is in the guide I linked to or on the Via Lusitana's website.
Bom caminho, Laurie