Square Enix
has announced that Final Fantasy X & Final Fantasy X-2 have together sold over 14 million copies worldwide on PlayStation 2. We can assume X sold 8-8.5 million where X-2 sold 5.5-6 million. Here's a translation of the paragraph thanks to
goldpanner.
Quote
FINAL FANTASY X, released in 2001 for the PS2®, was the first in the series to employ character voicing, and the moving story and overwhelming graphics endure even today. In 2003 the first direct plot-continuing sequel in the FF series, FINAL FANTASY X-2 was released, and of these two titles together over 14 million copies have been sold worldwide.
Sadly we don't have have combined numbers for X/X-2 HD Remaster but we know it sold over 500,000 copies in Japan in less than a year and sold quite well internationally and even got a PS4 bundle in China after becoming a best seller, exceeding Square Enix's sales expectations. With all this in mind, FFX is easily the second best selling game in the Final Fantasy series and might even be in the conversation for the best selling ever. X-2 is also now the 4th highest selling Final Fantasy game in the series and would only be slightly behind VIII.