The blinds (and ante and limits and other values) traditionally change based on level, not on number of players. There's really no provision for changing them based on other conditions.
You could conceivably use the layout conditions to display a different ante based on number of players, but you're basically hard-coding it into the layout instead of the tournament settings. For example, you could change the "Ante" cell such that it has a Property Set that displays when the number of players is less than 7. In that Property Set you could set the Contents to be something like:
<nobr>Ante: 1 SB</nobr>
The conditions would be:
numberOfPlayers < 7
Then create another property set with Contents:
<nobr>Ante: 1 BB</nobr>
With conditions:
numberOfPlayers > 6
This should work, but depending on the layout, it might get more complicated. For example, if you're using the default layout, it changes colors based on level, so there are already property sets for Default (odd-numbered rounds), Even Rounds, and Breaks. You'd have to basically made an additional property set for each of those. In other words, add a second property set for odd-numbered rounds with conditions "numberOfPlayers > 6". Then add another for Even Rounds with conditions "isRound and (roundNum % 2 = 0) and numberOfPlayers > 6" and also change the existing "Even Rounds" property set's conditions to "isRound and (roundNum % 2 = 0) and numberOfPlayers < 7". Same for the other property sets.