UCC29002: The maximum parallel limit?

Hello Rock, 

Yes, there is a maximum number of units that may be paralleled, but it is possible to parallel more than 20 units.
The number is dependent on how low of a maximum Vcso value your application can tolerate. 

The true limit is the maximum drive current capability of the CSO output.  The leader module (with the highest Vcso output) drives the LS Bus of the system. 
Each UCC29002 controller on that LS Bus loads the bus with 100kR, including the leader's own load. 

Although the CSO output current is specified to be up to 1.5mA typical, it has a minimum guaranteed value of 1mA and that is the limiting number to design with. 
The highest Vcso is applied across all 100kR loads in parallel, so Vcso(max) = 10V can reliably drive 10 units in parallel, 5V(max) can drive 20 units, 3V(max) can drive 33 units, etc. 

The user designs the system so that Vcso(max) corresponds to the maximum current each PSU must contribute to the sharing. 
The main drawback is that sharing accuracy is lower as Vcso voltage gets lower.  So more modules can share current, but the sharing accuracy is not as good as fewer modules with higher Vcso. 
See UCC29002 Excel calculator tool: https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-GtQkXDHgDE/1.1/UCC29002_Design_Calculator.xlsx 

Regards,

Ulrich