Chord number in music are usually indicated by roman numerals, as shown below.
Here is the 12 bar Blues progression again, this time shown in roman numerals.
Although there will be variations later in the book, the basic order of chords in a 12 bar Blues is - Chord I (first four bars), Chord IV (bars 5 and 6), back to Chord I (bars 7 and 8), Chord V (bar 9), Chord IV (bar 10), Chord I (bar 11) and Chord V for the final bar (bar 12).
A good way to memorise the progression is to visualise it bar by bar. Study the progression shown above until you think you have it memorised. Then shut the book and try asking yourself questions like... what chord is in bar 6? (Answer-IV). What chord is in bar 9? (V), Bar 3? (I) etc until you can answer instantly.