The maximum you can pay into all of your ISAs combined in a single tax year is currently £20,000. This excludes a Junior ISA (JISA). There is also an annual limit of £4,000 on the Lifetime ISA (LISA).
Overall, this is a very generous limit and has increased significantly over the years.
When the ISA was born in 1999, the annual limit was just £7,000. There is stayed until 2008. From there, it rose steadily over the years. Then 2014 saw a decent hike which increased the limit to £15,000.
After that, it climbed gradually to £20,000 in 2017, where it has remained ever since.
So, if you’re maxing your annual ISA contributions of £20,000 per year, then you are very doing nicely. Go you.