I guess it will be next to impossible to get tangible sales figures for the number of RAM packs sold.
I did, however find a quote from Geoff Boyd (one of the owners of Memotech, who was a well-known vendor of expansion modules for the ZX81), that
At one stage Memotech was the largest purchaser of dynamic RAM memory in the world! In effect Memotech cornered the market for ZX81 (and Timex 2000 in USA) memory packs and were able to put the big buyers at WH Smith, Boots etc on allocation." By the end of 1984, some 250,000 Memopaks had been manufactured.
Given that Memotech was a "second source" (so, probably not the first choice by inexperienced buyers or when bought as a package), I would assume the volume of Sinclair original RAM packs about 3 to 5 times higher. (Even if Memotech's product was, at about the same price as Sinclair's or lower if on sale, the much sturdier and generally "better" product and Memotech offered expansions up to 64k.) Memotech was not the only second source for memory expansions for the ZX81, there were a lot more, mainly "cottage industry" sellers (like dk'Tronics, for example) selling memory expansions.
Overall, I would assume the percentage of memory-expanded ZX as at least 80%, very probably exceeding 90%. No one I knew who had the computer didn't also own one or the other RAM Pak. If you have a look into contemporary publications like ZX Computing (the main sources of software at the time), the vast majority of type-in programs listed needs 16k, and reader's letters very rarely complained about that.