Trying to enter patterns too many times will get your device locked. In most сases a message will appear saying that you will need to enter a PIN to access the device. If the tablet has a Google account attached, then you can send the PIN to your email. You can do that from the same window. After you send the email, check the inbox or the account that was attached to the tablet and enter the PIN received. Please notice that the tablet needs an active Internet connection for this to work.
On the other hand, if you don't have access to the account attached, you will need to perform a factory reset using the "three finger technique" which consists in pressing 3 simultaneous buttons to enter Recovery Mode. To do this, press Volume Down + Menu Button + Power Button for a few seconds and you will enter Recovery Mode. From that location, use the Wipe Data/Factory reset option and restart the tablet and you will have access to your device.