In a T20 match, if a fielding side has only ten players present on the field (either through injury or breaks, without bringing in a substitute), how many players can field outside the 30 yard circle after six overs?
We can't tell you because this isn't specified in the Laws of Cricket; you will need to refer to the specific playing conditions for the precise tournament(s) that you are interested in.
That said, we can look at (say) the playing conditions for the 2022 Men's T20 World Cup:
28.7.4 During the Powerplay overs only two fielders shall be permitted outside this fielding restriction area at the instant of delivery.
28.7.5 During the non Powerplay overs, no more than 5 fielders shall be permitted outside the fielding restriction
That seems pretty clear to me.