I want to test some software that spawns ~ 120 threads... is there a hard limit on any of your plans?

Linode accounts are, for all intents and purposes, separate servers. Once you are logged into your Linode, it functions nearly identically to a dedicated Linux server. So no, there are no process limitations vs what you could do with a dedicated server.

The thing you need to be careful about is memory usage. The facility for which there is most contention between Linodes is disk i/o bandwidth on the host platform. Linodes which swap excessively are penalised beyond just the slowdown that lots of swapping causes. Linodes have a token bucket limiter controlling their disk i/o activity - excessive swapping empties the bucket and restricts i/o activity to the bucket refill rate. Persistently hitting the limiter results in the bucket size and refill rates being reduced.

Pick a plan with sufficient memory to ensure that you don't have to swap out stuff that is active during the test - or be prepared to upgrade if you can't predict ypour memory requirements at the start.

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/ Peter