These mag carrier auto-retain almost every pistol magazine on the market and do not require any specifications during the ordering process. Most customers buy 1 Double Mag Carrier or 3 Single Mag Carriers per purchase, but getting just 1 to try out is perfectly okay too.
The two belt attachments that are most popular for mag carriers are the Quick Loop Attachment or the DM2 Molle Attachment.
If you're unsure what to order, the Quick Loop attachment offers the most versatility, including adjustment for belt width & cant angle and it's quick and easy to take on or off the belt. Attachments are not permanent, they can be easily changed later on.