Baby One More Time - 10,500,000 physically/digitally copies WW. With (SPS) 14,620,0000.
Sometimes - 2,000,000 physically/digitally copies WW. With (SPS) 2,830,000
(You Drive Me) Crazy - 2,650,000 physically/digitally copies WW. With (SPS) 3,600,000
Born To Make You Happy - 1,700,000 physically/digitally copies WW. With (SPS) 2,100,000
From The Bottom Of My Broken Heart - 1,500,000 physically/digitally copies WW. With (SPS) 1,760,000
Oops!...I Did It Again - 6,000,000 physically/digitally copies WW. With (SPS) 9,730,000
Lucky - 1,800,000 physically/digitally copies WW. With (SPS) 2,640,000
Stronger - 2,500,000 physically/digitally copies WW. With (SPS) 3,400,000
Don't Let Me Be The Last To Know - 700,000 physically/digitally copies WW. With (SPS) 900,000
My Only Wish This Year -1,500,000 physically/digitally copies WW. With (SPS) 3,160,000
I'm A Slave 4 U - 2,670,000 physically/digitally copies WW. With (SPS) 3,700,000
Overprotected (Original & Remix) - 1,900,000 physically/digitally copies WW. With (SPS) 2,590,000
I'm Not A Girl, Not Yet A Woman - 1,500,000 physically/digitally copies WW. With (SPS) 1,960,000
I Love Rock N' Roll - 600,000 physically/digitally copies WW. With (SPS) 1,000,000
Boys (Remix & Original) 700,000 physically/digitally copies WW. With (SPS) 1,080,000
Me Against The Music - 2,000,000 Physically/digitally copies WW. With (SPS) 2,470,000
Toxic - 6,500,000 physically/digitally copies WW. With (SPS) 12,340,000
Everytime - 2,800,000 physically/digitally copies WW. With (SPS) 3,900,000
Outrageous- 500,000 physically/digitally copies WW. With (SPS) 763,000
My Prerogative - 1,500,000 physically/digitally copies WW. With (SPS) 1,830,000
Do Somethin' - 1,800,000 - physically/digitally copies WW. With (SPS) 2,160,000
Someday (I Will Understand) - 350,000 physically/digitally copies WW. With (SPS) 400,000
And Then We Kiss - 200,000 physically/digitally copies WW. With (SPS) 217,000
Gimme More - 3,700,000 physically/digitally copies WW. With (SPS) 4,800,000
Piece Of Me - 3,900,000 physically/digitally copies WW. With (SPS) 4,960,000
Break The Ice - 2,000,000 physically/digitally copies WW. With (SPS) 2,470,000
Womanizer - 7,650,000 physically/digitally copies WW. With (SPS) 10,310,000
Circus - 6,550,000 physically/digitally copies WW. With (SPS) 7,980,000
If U Seek Amy - 2,200,000 physically/digitally copies WW. With (SPS) 3,030,000
Radar - 1,500,000 physically/digitally copies WW. With (SPS) 1,980,000
3 - 4,600,000 physically/digitally copies WW. With (SPS) 5,350,000
Hold It Against Me - 3,650,000 Digital Copies WW. With (SPS) 4,420,000
Till The World Ends (+ Remix) - 5,700,000 Digital Copies WW. With (SPS) 7,480,000
I Wanna Go - 3,200,000 Digital Copies WW. With (SPS) 4,310,000
S&M - 1,500,000 Digital Copies WW. With (SPS) 1,845,000
Criminal - 900,000 Digital Copies WW. With (SPS) 1,800,000
Scream & Shout (+ Remix) 9,300,000 Digital Copies WW. With (SPS) 14,500,000
Ooh La La - 1,470,000 Digital Copies WW. With (SPS) 1,700,000
Work ***** - 2,200,000 Digital Copies WW. With (SPS) 4,900,000
Perfume - 750,000 Digital Copies WW. With (SPS) 1,250,000
Pretty Girls - 850,000 Digital copies WW. With (SPS) 1,796,000
Make Me... - 1,450,000 Digital Copies WW. With (SPS) 3,370,000
Slumber Party (& Remix) - 450,000 Digital Copies WW. With (SPS) 1,436,000
Private Show - Has sold over 330,000 units WW with 100,000 being from Digital sales.
Do You Wanna Come Over? - Has sold over 450,000 units WW.
Total Physical/Digital Single Sales Worldwide - 105 Million Singles
With (SPS) 170 Million Units.
Baby One More Time is her best selling single, and her best selling single of the 90s.
Toxic is her best selling single included with streams of the 00s. But Womanizer is her best selling song Physically/digitally wise (also 00s)
Scream & Shput is her best selling single of this decade.
Till The World Ends is her best selling single as a lead/solo artist.
Britney has over 4 songs with over 10M Units WW, and over 7 songs selling over 5M units.
Britney also has 39 songs sold over 1M units WW including the (SPS) Method. 33 being from pure.
She has way more sales but I can't be asked to count non-singles so here you go