How much pressure fluctuation is normal on 3/4" line when one tap is open?
There are too many variables involved; it isn't really feasible to give a specific answer. You can determine an answer for your situation by determining the main supply line pipe material and length, checking for excessive quantities of elbows (ie more than just a couple), and then consulting a pressure loss table. For example, The Engineering Toolbox web site has such tables for SCH 40 PVC and Types K, L, and M copper tubing. Similar tables may be found for PEX, IPS- or CTS-sized HDPE, and other tube materials.

In all honesty, though, you can probably do well enough using one of the copper tubing tables even if your supply line is another material. For example, Engineering Toolbox provides a chart for Type K copper. The purple line 5th from the left is for 3/4" size. Choosing an arbitrary point on the line we see that expected pressure loss is about 0.1 psi/ft at 9 GPM flow. If you had a 100 foot supply line we'd expect 10 psi loss with 9 GPM flow. A static pressure of 100 psi and a loss of 10 psi while flowing should provide plenty of pressure left over for the regulator to be able to maintain a setpoint of 60 psi inside the house.

If that's not the case, the first thing you could do to investigate is to install pressure gauges before and after the regulator. You'll be able to see the static and dynamic pressures on both sides of the regulator and immediately determine whether you have excessive pressure drop in the supply, across the regulator, or possibly in downstream plumbing in the house.

In theory a regulator should hold its output steady through any fluctuations in flow or in supply pressure. In practice it won't be perfect. Sudden changes in flow will cause the regulator output pressure to overshoot or undershoot a bit (maybe a few psi I'd guess), and higher steady flow may also cause the regulator output pressure to run a little low. Each regulator is rated for a specific amount of flow; below this limit it should regulate fairly well. In all cases if the supply pressure drops too near to the output setting, or even falls below it, then all regulation is lost and the output pressure becomes equal to the input minus some semi-constant amount.