Do software companies tend to hire more employees than needed?

In general no. Headcount is their primary expense and actually labor management relations are tough in the industry because of that. Unlike other industries that make capital investments to save labor, the software industry's only way to control costs is play hardball with the help.

Google, Facebook, and Microsoft though are an exception to that. With monopoly businesses they are highly profitable but very poorly managed. Google kills products that everybody loves, Microsoft kills products by advertising them in an annoying way whenever you move your mouse, and Facebook can't kill the cringiverse no matter how much everybody hates it. You wouldn't look to Adobe or Oracle to learn how to develop better software on budget and you shouldn't look at "big tech" that way.

Sometimes?

When I worked at a small company we had two java guys who basically sat around for a year because they were hired speculatively and we didn't end up using java, and the general manager hired a sales guy for the americas who never did anything and just sat in his office the entire time for two years.

I think it depends on what the priorities of management are at the time. Sometimes money isn't the biggest issue and management may be very weak at examining productivity so you'll have excess staff.