Note: This is a duplicate of: https://gaming.stackexchange.com/a/198417/153355

However, here's the answer:

You're scanning for the nearest player, your should scan for all the players.

Solution

/testfor @a[x=X,y=Y,z=Z,r=R]

Where:

• X the x location
• Y the y location
• Z the z location
• R the radius

For example:

/testfor @a[x=64,y=10,z=64,r=5]

(Search for players on coordinates 64,64 at height 10, with a radius of 5 blocks)

Variables

Specify the location:

• @p nearest player
• @r random player
• @a all players
• @e all entities

Specify the values:

• x, y, z coordinate
• r, rm radius (max, min)
• m game mode
• c count
• l, lm experience level (max, min)
• score_name max score
• score_name_min min score
• team team name
• name entity name
• dx, dy, dz volume dimensions
• rx, rxm vertical rotation (max, min)
• ry, rym horizontal rotation (max, min)
• type entity type

Examples:

To test if Alice is online:

testfor Alice

To count the number of players in survival mode within a 3-block radius of (0,64,0):

testfor @a[0,64,0,3,m=0]

To count the number of players currently flying:

testfor @a {abilities:{flying:1b}}

To count the number of zombies within a 20-block radius of (0,64,0):

testfor @e[0,64,0,20,type=Zombie]

Source: https://gaming.stackexchange.com/a/198417/153355