Just came across this, so why did Grimmjow lose his number when he lost his arm and why was some arrancar kicked out of the espada
Grimmjow lost his number because he had gone against the orders to stay in Las Noches that were issued by Aizen. Kaname dealt the punishment that he felt would be worthy for such rebellion. Some Arrancar were demoted from the rank of Espada due to simply being weaker than the more recently created Arrancar. The Privaron Espada are such an example. They were once part of the Espada, but were demoted due to the stronger Arrancar replacing them and their status as Espada levelled Arrancar.
Here's a wiki entry on the Arrancar you're referring to.
Privaron Espada
The Privaron Espada are former Espada who have officially lost their rank and are assigned a three-digit number. Being former Espada, they are far stronger than the normal Números. They reside in Tres Cifras (3ケタ (トレス・シフラス), toresu shifurasu; Japanese for "Three-digit", Spanish for "Three Digits"). As mentioned by Dordoni Alessandro Del Socaccio, these former "naturally created" Espada were rendered obsolete after Aizen began artificially creating Arrancar to meet his own standards.
Simply put, they were child's play compared the newly created Arrancar that Aizen had created via use of the Hōgyoku. However, they are more powerful than Números Arrancar.