Depends a great deal on what I am doing, who I am with and the boat I am in.
For me it depends entirely on which boat, type and width and seat height, and where, flat or moving water or open-water wind. The “with whom” only comes into play on family trips; my wife often refuses to carry a spare paddle in her solo. So I bring two spares.
I always have a double blade (I know. . . . ), usually a long, lightweight carbon double.
I always have a single blade, but depending on the boat design and trip that could be anything from a 48” to 52” bent shaft to a straight single, the latter anything from a beastly heavy unbreakable Sawyer 50” straight to a 54” BB Expedition Plus to (mostly for memory reasons) one of my dad’s massive guide sticks.
If I am not portaging, and have a spray cover with paddle pockets, I may bring a double, a bent shaft and a straight. And a sail, and a 6’ long push pole/hiking staff/spare tarp pole. Why not, there are paddle pockets both bow and stern.
P2160520 by
Mike McCrea, on Flickr
Having a spare paddle seems essential gear to me, and switching off, for varying conditions or just swapping for a different feel/different stroke motion, is enjoyable. To that end my spare paddles are, um, different in some way from my primary stick, usually heavier and (I hope) of more durable construction.