I was originally referring to the common sizes (and forgetting which was which!).
It's easy to find out what you have, with bolt and a decent ruler: count eleven ridges along the bolt, and measure the distance. Do twenty if it's a small bolt!
Most of the commonly available stuff is metric 'coarse' pitch. As Gerry says, there's usually a fine pitch defined, too, especially for the larger sizes. They're rarely seen in DIY/construction, but I've come across them in the motor industry, for example the M10* bolts securing seatbelt webbing straps in our old Passat were fine pitch. The reasons are obvious - extra strength and less likelihood of unwinding by itself.
Similarly, there are uncommon metric bolt sizes. For example M7 is defined but hardly ever used in everyday applications, and I was amazed to find it was used for the battery clamp bolt on daughter's Rover 200. I now own three M7 bolts (always have spares!), which weren't cheap and had to be sought out locally, but I replaced the fitting with M8 (which everyone else uses) at the first opportunity.
People used to grumble about the range of non-metric threads, but they all had a purpose. Some were easily done up/undone (Whit.), some had strength (BA, UNC, UNF) - you could use the right thread for the task. In a metric world one size fits all, unless you're in manufacturing.
E.
*might have been M12 - it was a long time ago. I still have the bolts somewhere...