In the US, I'd say around 2/3 to 3/4 of new production boltguns larger than .223 a fixed mag. For small calibers it's more or less an even split, and detachable mags are a lot more common in foreign rifles.
Bear in mind that gun stores tend to lag a few years behind production and Remington is still phasing out fixed mags, so if you went down to your local store the ratio might be a bit higher.