"A lot" is a relative term. None of us can really comment on how well GTS is
actually doing since we have no clue how big of a budget it had nor do we know what Sony set as a sales target. Not to mention the only sales stats we seem to have are for the UK and even then we don't get solid numbers.
However what we can do is roughly compare GTS to other Sony first party titles using estimated sales numbers and in that aspect it's not looking so good. As of May GTS is
estimated to have sold 3.3 million copies which may seem great, until you look at the other titles. God of War is a certified monster and
has passed GTS in less than a month, Horizon Zero Dawn reached 7.6 million around it's anniversary and the last Uncharted was at
8.7 million at the start of 2017.
Considering how long GTS took and the likely timeframe of the next one being similar, I'm not so sure 3.3 million is as big of a sales number for a well-established first party title.
I'm sure lots of people also have secondary accounts they use on occasion for various purposes. If they ever fired up GTS on that account it's considered a completely different person. Those probably boost the numbers quite a bit as well.