(Writing as someone who lives part-time in central Portugal)
The bigger towns/smaller cities: the likes of Santarém, Tomar, Coimbra etc. have quite a lot of taxis. They typically have what North Americans would call a "stand" near the railway station if there is one, or the inter-community bus stop, or somewhere central. You also can hail them on the street.
Smaller places might have one taxi, or two, maybe--in our town, Vila Nova da Barquinha, we have two owned by the same family. Both adults drive cab.
Usually, all the cafés and restaurants will know who the local cab drivers are, and can call them for you. Drivers seldom speak English. In Entroncamento, our nearest larger town, there's maybe 10-12 taxis; we've pretty much met all of them over the last few years as they deliver us the "last kilometer or two" from the railway to our house. I think 2, maybe 3, of that 10-12 speak some English. With the rest, until our Portuguese improved, it was generally "name the place and hope for the best." No bad surprises so far.
In our little town, of the two family drivers, one speaks only Portuguese (the husband); the wife also speaks French because she worked in Switzerland for many years. She and I have an arrangement because as a Canadian of "a certain age," I had to study French quite a bit in school. She speaks to me in French, I'm supposed to answer in my really bad Portuguese unless I can't think of a critical word; then she corrects my Portuguese. Works for us!
Very often older Portuguese--and many of the drivers are close to what we would consider retirement age, have worked in other European countries or have come back from North America. So if English doesn't work, and you don't have Portuguese, you can always try French (quite common) or German or perhaps Dutch. Spanish--not as likely.
If I remember correctly, cab fares in the towns right now start at 4 euros 25, and stay at that till the outskirts of that town. Then the fare goes up in .10 increments. Our fare, for about 4 km outside the town, usually runs about 7 euros 50. There's a slight surcharge after dark. There is also a slight surcharge, maybe 50 euro cents (half euro) for big bags in the trunk. They generally insist you put backpacks, unless very small, in the trunk.
I think rates are fairly standardized across the region, but they may have gone up slightly since February, when we were there, just because fuel is going up. But they're generally a quite good deal, because a lot of the locals without cars (the elderly, mostly) use them consistently for things like doctors' appointments.
I have always found the cab drivers very agreeable.
