I am working on a poster in which the background is a photo that I took myself (18 MP). I am wondering what would be the best resolution for printing this photo on an A1 canvas. This might sound very basic, but I have no experience in printing photography at all.
For this size 150ppp is already a good resolution.
A1 is 33.1 in × 23.4 in you need (x150) => 4965x3510 pixel. That is 17.4MP, so if you use the full image (no crop) you'll be fine.
The best setting is going to depend on the printer, but use the highest resolution you have. Any decent printer should support decent built in scaling, you just need to make sure your pixels per inch setting is correct on the image format if the printer doesn't automatically scale to the print size in the printing dialog.
You can try to artificially increase the DPI/PPI in Photoshop which might give slightly better results than the build in scaling, but the best setting to choose would depend on the printer's native resolution (which often isn't even the same in both directions.)
The 150/300dpi rule is more for what quality level you need for a large image to look decent. It's an arbitrary number though and isn't some idealized resolution for all printers. It has more to do with the resolution necessary for the image to appear smooth to an eye viewing it. It is also highly dependent on viewing distance. A billboard for example doesn't even need close to 150ppi because of how far away the viewer is. If your image is smaller than this rule before scaling, scaling is only going to make a very limited difference and probably only noticeable if you also do sharpening on it after the fact. In your case, as long as people will be viewing from a meter or two away, you should be alright at 150.