How much thrust is applied in a 737 to achieve taxi speeds?
From the 737 (Classic and NG) normal procedures section of the flight manual:

Aircraft response to thrust change is slow, particularly at high gross weights. Idle thrust is adequate for taxiing under most conditions. A slightly higher thrust setting is required to start taxiing. Allow time for aircraft response to each thrust change.
To initiate taxi, release brakes, smoothly increase thrust to minimum required for the aircraft to roll forward and reduce thrust to idle. Under normal conditions, 45% N1 should be considered the maximum N1 for breakaway thrust. Do not start a turn until sufficient forward speed has been attained to carry the aircraft through the turn at idle thrust.

In short, advance the levers a little, achieve breakaway, then back to idle.
Smaller jets with higher thrust-to-weight ratios, such as business jets, often taxi with one of the engines in idle reverse, instead of having to ride the brakes.