As with most of the packing elements, personal preference and time of year will really determine what you'll need. I walked the Via de La Plata in the late spring and early summer (May-June). The weather was comfortable and I chose two T-shirts and two long-sleeve fishing shirts. If I'd walked in the fall or winter, I'd have wanted something different, I suspect.
For most of the trek I dressed in layers. The mornings were a bit cool and sometimes dew ridden. Then, as the sun rose, off went the jacket. And, by late afternoon, sometimes the long-sleeve shirt went, too and I walked just in a T-shirt.
The T-shirts also doubled as sleep shirts for me. I'd take my afternoon shower, dress in a clean T and then later that night sleep in that shirt and be partially dressed for the next day.
I think the concept of layering is a good one. Being able to add and remove layers helps. But, you also don't want to pack too much (that's why I chose to use my T's for multiple tasks.
The reason I chose the Columbia long-sleeve shirts was first because of the pockets. It's always good to have pockets when walking a Camino. Also, the shirts are designed to breath with vents in useful places. I also liked the buttons that allow you to roll up your sleeves and keep them in place with hooks of fabric and buttons. Like the idea of layering, rolling sleeves up and down give you more options for comfort (and style). Plus, they wash and dry very easily. I'm still wearing these shirts, they hold up great.
No matter what style of shirts you choose, I highly recommend some sort of tech-based materials. You want shirts that wick away moisture, that allow your skin to breathe, that dry quickly, and there are even materials now that cut down on odors (really useful during weeks of walking and sweating and handwashing).
It is worth noting that you'll mostly be handwashing your clothes, so you want fabrics that will stand up to ringing like that.