If you check Construction Enquirer on line, it states that Kingscote Construction are the main contractor for the redevelopment of the North Stand. It mentions that the North Stand's current capacity is 6,537, increasing to 12,516. The extra seating to the Trinity will push the overall capacity above the required 50,000.
The new North Stand will have about 1,000 less seats than the Holte End.
Interestingly, the new North Stand will still be bigger than Bournemouth's Vitality Stadium, which only holds 11,286!