The ideal inflation for most soccer balls is 6 to 8 PSI (lbs). The recommended inflation spec is also usually list next to the valve on the ball. Inflating a soccer ball too much or too little can damage it and affect play. Always moisten the inflation needle before you insert it into the valve. Preferably, use some silicon oil to moisten the needle.
Balls must be pumped regularly to keep them inflated. Latex bladders, especially, require more regular pumping than butyl bladder soccer balls.