It all depends on the dynamics at the tables. Sometimes I can feel normal at four tables or more (I do not use additional programs), and sometimes I do not have time to think about my decisions playing on two tables ..
Also in different rooms people play differently ....It also affects how many tables I can play at one time.
Basically, these are 4-12 tournaments per day, depending on the mood, but not simultaneously !!! Some I lose then add new ones !!! You can play even in a million tournaments but if you don't know how to play you will never win a tournament !!!
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