50 (or more) channels picoscope/oscilloscope
The reason you can't find a scope with enough channels is because you're looking for the wrong tool. Scopes are best for analog stuff, and ones with more than 4 full-featured channels are rare. Each channel on a scope has a relatively complicated, and thus expensive, high-bandwidth low-noise amplifier, so a scope with 50+ channels would be extremely expensive.

For a digital signal, you don't need the analog front-end, you just need a comparator for each channel.  A scope-like instrument with lots of digital inputs is called a logic analyser. 32 channels is pretty common, and 50+ channel versions aren't too hard to find.  As a bonus logic analysers are usually better than scopes at decoding the signals and displaying higher-level protocol-dependent information.

One thing to watch out for is that the logic analyser can't tell you as much about a signal which is wrong.  It can't tell the difference between a signal which is sent wrong, and one which has been messed up by interference.  So you still need the scope.  Use the logic analyser to identify which signal(s) is/are not what they should be, then look with the scope to see if the problem lies in the digital logic or in the signal integrity.