For the Radical Firearms BTR 7.62x39mm with the 10.5" barrel, the answer depends on the specific SKU and distributor.
Most Radical Firearms factory listings show the pistol shipping with one 10-round 7.62x39 magazine.
Some distributors have sold the BTR-branded package with two 10-round magazines included.
As for magazine compatibility:
The magazines are not proprietary. The firearm uses a standard AR-15 lower receiver with magazines designed specifically for 7.62x39 AR-platform rifles/pistols.
Higher-capacity magazines are readily available. Many owners run 20-, 28-, and 30-round 7.62x39 AR magazines from companies such as C Products/Duramag. In fact, one Radical owner specifically mentioned using C Products 7.62x39 AR magazines to improve reliability.
Finding higher-capacity magazines is generally not difficult, though 7.62x39 AR magazines can be a little more finicky than standard 5.56 AR magazines because of the cartridge's taper. Quality magazines matter more with this caliber.
If you're looking at a specific retailer listing, send me the SKU or link and I can tell you exactly how many magazines that package includes and which higher-capacity magazines tend to work best in that particular model.

For the Radical Firearms BTR 7.62x39mm 10.50" pistol, the magazine details are as follows:
Capacity & Count: The pistol typically ships with one 10-round magazine from the factory. Some distributor packages may include two 10-round magazines.
Proprietary Mags: The magazines are not proprietary. The pistol features a standard AR-15 lower receiver design and utilizes 7.62x39 AR-pattern magazines.
Higher Capacity: Various manufacturers produce magazines for this platform in higher capacities, such as 20 or 30 rounds. These are generally available through major firearm component retailers.

Hi Richard and welcome. No, the magazines are not proprietary for the Radical Firearms BTR 7.62x39mm 10.50" Barrel. It uses standard AR Pattern 7.62 x 39mm Magazines and Drums which are very common and easy to find. Buds has plenty in stock here: budsgunshop.com/search.php/q/ar15+7.62x39 ----------- Cheers, stay safe and enjoy! P.S., maybe if enough of us make the suggestion, Bud's would add a "Reply" feature as it would make questions and answers much easier: budsgunshop.com/contact_us.php?printForm=reportError