The current night-game ordinance allows the Cubs to make up a postponed game as a night game, if the original game was a night game, which this one was; it doesn't change the number of night dates for this season.
It does deprive the Cubs of what was to have been an off day before the St. Louis Cardinals come to town for a two-game series May 7 and 8, and turns what would have been a nine-game homestand with an off day, into a 10-consecutive-day homestand (four against the Padres, three against the Reds, the makeup vs. Texas, and the pair with the Cardinals). The Cubs' next off day, then, will be the day after the two-game Cardinals set, before they have a weekend series in Washington May 10-11-12 against the Nationals.
The Cubs haven't yet released ticket info for the makeup date, but the original date was a "silver" level date, and the games surrounding May 6 are "gold" level (likely due to the Reds and Cardinals as opponents). We'll see how they price this game. If you have tickets dated April 17, though, they will be good for the makeup game.
The Cubs also made this announcement today:
So their desire to play some Friday games at 3:05 is postponed... until at least July.