Hi and welcome to the forum.

For most countries, the issue of "tax residence" is determined by "preponderance of evidence" - which means that the rules are a bit open to interpretation and depend on how interested the local government is in adding to their tax lists. For most countries, 183 days spent in the country is enough for them to "presume" that you are resident there. (Translation: It is possible to claim residence elsewhere, but you'd better have some pretty compelling reason.)

The official determination is supposed to rest on where you have your "primary center of interest." This includes where you spend the most time, where you return to on a regular basis to sleep, where you own a home, where you perform the work for which you are paid.

In your case, you should be ok as long as you aren't spending more than 183 days a year in Portugal - but the criteria is not always very clear cut and the Portuguese could raise the issue if you appear to be "residing" in Portugal.
Cheers,
Bev