1-4 clamps can be used (consider 20 minuted for the glue to dry.)
It is recommended (obviously) to use 4 clamps around a picture frame to save time, keep precision, stability, even distribution of clamping pressure, alignment bonding etc.). and more in details below:
Some framers buy a set of 4 clamps, other buy one or two clamps to get familiar/experiment with MasterClamp mechanism, technique, handling, performance, efficiency, precision and quality. Based on test results of end product they would typically go on to get a complete set of 4 clamps (for obvious reasons) MasterClamp provide the world's best framing clamps and clamping experience.