As mentioned above, most people apply to more than one school at a time. This is largely driven by the fact that there are so few rounds (two "good" rounds for most schools, round one and two), thus you pretty much have to apply to multiple schools at a time unless you only plan to apply to two programs. That said, clients often split applications across rounds, doing three in the first round and two in the second round for example.
In terms of the actual timing within the round, I recommend that applicants try to focus on no more than two applications at a time. It is very possible to do them one at a time also, which still allows you to get in the mindset of one program, finish the application and move onto the next if you are efficient.
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