Depends on the article size
500 words = 2 internal links
1000 words = 3 internal links
2000 words = 4 internal links
And so on...at least it's what I do
i'll second that,
but i'll go with more, 6 for 1000 words & 12 for 2000 words, probably more if it fits,
there's nothing wrong with adding more as long as they're natural and engaging,
and you're keeping a good overall links/page ratio for each of the pages depending on their weight,
there are plenty of plugins & services that can help you keep track of these,
internal links will help passing link juice to all of the pages without affecting the source pages,
they could be helpful for user engagement & overall UX signals, which do have more importance after the latest updates,
and last to mention is the crawlability of the website, they'll get your pages crawled faster, more often and that's something you surely want,
google can crawl up to 150 links per page,
i would limit the overall links from a page to 100 for any website with decent authority
and 50-70 for relatively new websites,
now, let's say you have 30-40 links for navigation, social media, etc... (they're usually less, but well),
that leaves you with 20+- contextual links (internal & external), so just don't spam it & do it whenever you have the chance