Is there like a set list of things you need? And can there only be 7 at one time?
The Shichibukai, the full title being Oka Shichibukai (王下七武海 Ōka Shichibukai?, literally meaning "Royal Seven Military Seas"), are seven powerful and notorious pirates who have allied themselves with the World Government.
So to answer your second question, Yes they can NEVER be more than seven. But mostly there are less than seven because of inherent nature of Pirates. Law, Blackbeard, Crocodile etc. are former warlords who joined just for furthering their own goals. The current members of seven warlords are Mihawk, Kuma, Hancock, Buggy and Edward Weevil.
Law and Doflamingo have left their positions after the events in Dressrosa.
To become a Shichibukai several criterias maybe looked upon but the final decision is taken by the Gorosei (Citation needed, but Gecko Moriah's termination etc. points to this direction).
- A space must be vacant for a new Shichibukai to be selected. If there are already seven no new names would be considered.
- A exhibition of strength against pirates is a must. Example. Blackbeard handed over a 550m Bounty Ace to the World government. Law gave 100 hearts of pirates. This is seen as a sign of pledging loyalty to the World Government.
- Since Shichibukai are used as a tool of intimidation, the shichibukai are usually well known pirates with high bounties and potential, Blackbeard being a notable exception. Buggy despite being a weak pirate was recognized as a warlord because of the following he generated as well as his name being associated to Pirate King Roger and Yonko Shanks.
However, once admitted Shichibukai have some duties that are expected from them in order to continue as a Shichibukai. So if they don't do this their bounties may be re-activated.
- They can't show open defiance to World Government. They can attack only other pirates and may be needed to pay some "tax" to the government. They shouldn't be defeated since this will malign their reputation which is one of the main requirement to intimidate other pirates.
- They may be asked to do some tasks for the government, like fighting in wars (Example: Boa Hancock may have lost her seat as a Shichibukai if she declined to fight at Marineford Summit War), liberating islands etc. But their main purpose is to fight against other Pirates.
For more information of benefits of Shicibukai and how they are dismissed etc. refer to the wikia, One Piece - Shichibukai. There are some uncited claims so take them with a grain of salt.