Currently there's up to 21 volumes of Tensura (short form of this series) light novel (in Japanese, of course). There's still expected to be at least one more volume left. At LEAST.
There's official translation LN that you can buy both physically and digitally. For physical version, you need to look it up yourself if it's available at your nearest Kinokuniya etc. For digital version, I found that there's up to 19 volumes available at Amazon.
As for fan translations, you can find them at:
https://tensurafan.home.blog/light-novel/
However they only have from volume 6-20. So not all volumes are available. You can either look for them at the all-novel-you-can-look-for site:
https://www.novelupdates.com/series/tensei-shitara-slime-datta-ken-ln/?pg=2#myTable
Or, look for them yourselves, like in reddit.
Does anyone know where i can read the novel please?