1) Whether MX104 is capable of receiving full routes from two different ISP .
Yes, if the only major purpose of the device is dual peerings and a merged internet table the device should be fine for the services. But as Python as mentioned this routing capacity and management will be a significant percentage of the FIB so I would not load a number of other services on the device.
2) Maximum routes that can be learned by the MX104.
FIB 1 million RIB 4 million with the current internet active table being under 700 k right now.
3) Best practices or precautions that need to be taken before receiving
For this type of setup, I would establish the sessions one at a time and add the second one after hours. During convergence routing paths will change and can have minor effects on existing flows to internet sources. But this is a pretty simple setup without much complication.