A woman that made a Hefsek Tahara and counted for 6 days, but then didn’t count the 7th and didn't go to the Mikvah since she was not going to have sexual relations. Another 10 days passed.
Can she count the 7th day and go to the Mikvah?
The Poskim in Yoreah Deah 196:4 write that there is not allowed to be more than a 5 day gap in the middle of the shiva nekiyim.
She must start the 7 nekiyim again. However,according to some, a hefsek tehara may not be required.
She needs to count the seven consecutive days and then she can schedule her mikvah bath. To avoid these kinds of mistakes you can use MikvahCalendar App which will simplify mikvah calculations and also send you reminders for that.