i just started doing longer combos, my longest was 15 with tatw ending but i still can only manage like 6 rick combos on a regular even that.
i practice by starting with like 10-15 atw in a row, combo. try and get as high as you can. so if u think like u can do 6 or something do like 8 twice before moving on, this will strenthing your legs. and do the same both directions both legs.
then do hatw back and forth and if u think you can do 6 then get 8 twice or three times. this will strengthing ur hatw. and same for htw. till
try to get each one at a high amount and then start will small combos such as htw atw. then juggle and do it again like 10 times
and switch up to like atw hatw and do it like 10 times then swithc to other short combos.
then try some longer ones. good luck