I've just checked on a 2021R2 upgrade for you. You can definitely have more than 20 on display.

I’ve managed this for a customer having 20+ stages for cases

Try this, just change a param on custom_sysparms and the refresh metadata

You can check the current value whit this query:

select * from Custom_SysParams where parm_name = 'MaxPipeLineStages'

And Update to 30 states with this query:

update Custom_SysParams set parm_value = '30' where parm_name = 'MaxPipeLineStages'

It worked for me with versions as old as CRM 2018.

Note: Be very careful updating values directly on database, backup it previously!!!

As Tino Jovani points out, this SysParam value is only considered for the cylinder view, when set to rectangle the MaxPipeLineStages seems to be ignored.

Worked a treat. Thanks.