I think that you can have as many as you want, though if you're in Texas, it'd be technically illegal to have six or more:

https://en.wikipedia.org/wiki/Texas_obscenity_statute

In 1973, the Texas Legislature passed Section 43.21 of the Texas Penal Code, which, in part, prohibited the sale or promotion of "obscene devices." The statute defines "obscene device" as "a device including a dildo or artificial vagina, designed or marketed as useful primarily for the stimulation of human genital organs." The legislation was last updated in 2003, and Section 43.23 currently states, "A person commits an offense if, knowing its content and character, he wholesale promotes or possesses with intent to wholesale promote any obscene material or obscene device."[1] Section (f) of the law also criminalizes the possession of six or more devices (or "multiple identical or similar" devices) as "presumed to possess them with intent to promote."[1]