Guessing power line voltage from the number of disc insulator

In general, probably not.

25KV Low Voltage Silicone Rubber Composite Tension Suspension Electric Insulator for Railway

Low Voltage 220kV Polymer Composite Power Line Insulator

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In a specific country, with a single supplier of high-voltage transmission tower systems, for a small group of towers built at similar times, the answer may be yes.

Is there a good way to guess the power line voltage based on the number of insulator discs that attach the line to the transmission tower?

We usually use as a rule of thumb for determining the number of porcelain/glass suspension disc insulators: 10kV for each disc (standard 5-1/4' x 10").

For typical system voltages in North America, this would be:
69kV: 4-6 discs; 115kV: 7-9 discs; 138kV: 8-10 discs; 230kV: 12 discs; 345kV: 18 discs; 500kV: 24 discs.

You may check Electric Power Generation, Transmission & Distribution by Grigsby, et al.

See attached Table 4-6 from the USDA-RUS Bulletin 1724e300.