Assessing the impact of a single stay on a pole can be done using static analysis. Add the stay, select the pole, and view Properties panel > Structure in the sidebar:

An accurate analysis of both single and multiple stays is done using Finite Element Analysis (FEA).

Add the FEA panel by clicking the (+) next to the Properties panel in the sidebar, and checking FEA in the popup. Click anywhere outside the popup to dismiss it:

Select the pole(s) to simulate, and on the FEA panel scroll down to Summary Reports and select either Ruling Span or Network Solve under the Load Type section:

In the Simulations section at the top of the panel, click Run for each environment that you want to simulate:

To view the analysis reports after a simulation has been run, expand the Reports section of the panel, and then click to select objects next to each category to see the analysis:

Make any necessary adjustments to ensure the pole is within loading limits.

See also: Finite Element Analysis