"Do I need to have a permanent dance partner to be good?"

Nope! It’s an advantage to practice with different partners, actually. On courses, you often change partners regularly precisely to learn how to adapt to different people. Having a regular partner (perhaps someone you attend courses with regularly) can be nice and safe, but make sure you don’t only dancing with each other. You will develop faster by also dancing with others – some are taller, lower, faster, slower, more experienced, less experienced than you. This teaches you to lead/follow better. That said, if you have a friend or girlfriend who also wants to learn, it can be great to practice together at home between classes. Just don’t be afraid to ask others up to dance on socials even if you have a “go-to” partner. The salsa communities are generally very open and inclusive, and it is common to alternate.