Heres some words i picked up from wikipedia

As of 2000, some minority reports claim the Texas Syndicate had about 19,000 members in prisons and jails statewide with more on the outside. However, such numbers are often inflated and include inmates only marginally connected with the gang as well as ex-cons, most of whom do not remain actively involved.

is that mean other estimated memberships arent so much realistic,too?

Simon Chen, Wikipedia is generally a poor source on criminal groups and anybody can add their information to it. Texas Syndicate is a large ( everything is bigger in Texas ) prison gang throughout but the updated estimates are 4,400 members in total for prisons & streets.
The Source : https://www.dps.texas.gov/director_staff/media_and_communications/2014/txGangThreatAssessment.pdf

Generally speaking prison gangs are more exclusive/selective on their membership but some street gangs are too.

It the same things for the Artan Brotherhood some say 10,000 members in e out of jail but in real there are 200-300 full members in prison and thousands of associates in and out of prison that help to carry murders of sell the drug but don't was a full members.

The name Aryan Brotherhood is used by several different and completely separate organizations. The most notorious of these are the California and Federal branches.

La Eme in SoCal have around 155-300 members estimation.