This camera's user manual is available online.
On pp. 19 (overview) and 248 (quick reference) it shows the full viewfinder display with a gloss explaining every part of it. In particular, the number on the lower right is labeled "max. burst". Additional information about this reading is found by searching the manual for "burst"; useful pages include 70, 72 ("...maximum burst will vary depending on the subject, card brand, ISO speed, Custom Functions, and other settings."), 73 (explaining that the max. burst display cannot show values greater than 9), 102 (max. burst is less during WB shooting), 193 (with strong ISO noise reduction, max. burst will "greatly decrease"), p 220 (fine detail increases file size which can reduce max. burst rate).
From this information we can infer that the burst rate bottleneck is due both to on-camera processing speed (depending on noise reduction and file size) and transfer rates to the card.
Similarly, referring to your owner's manual will explain the focus lock settings.
You can get a lot out of your camera by reading through this manual and referring to it often.