Any time you make a purchase at a store that stocks >10-round magazines for your handguns and rifles that can accept them (otherwise known as "high-capacity magazines" by morons and "normal-capacity magazines" by people who know what those firearms were designed to use*), buy at least one. If your budget can only afford one, that is shiny. If your budget can afford more, buy them.
Even without touching on the mind-numbing stupidity of trying to ban these magazines, the simple truth is that mags are disposable items. Oh, sure, when those "disposable items" sometimes cost up to and including $50 a pop, you really do not like thinking about them in those terms, but bodies crack and bend out of spec, springs wear out, and followers do Hephaestus-knows-what in there, so it is always good to have spares on-hand should you need them.
And it is even better indeed to thumb your nose at petty authoritarians at every available opportunity.
(* – There are, of course, exceptions (for example, the Ruger 10/22 platform was originally designed for 10-round magazines), but they are rare.)