12 chairs row

12, 24, ….., 192, 204, 216, 228, 240, 252, 264, 276, 288, 300

Actual chairs (+4)

16, 28, ….., 196, 208, 220, 232, 244, 256, 268, 280, 292, 304

14 chairs row

14, 28, ……, 196, 210, 224, 238, 252, 266, 280, 294, 308

Actual chairs (-4)

10, 24, ……, 192, 206, 220, 234, 248, 262, 276, 290, 304

From the above, you can see that only 220 coincides with the actual chairs. So, the actual number of chairs is 220.