Performed below steps
1.Created new custom field called External Links
2.attached external system objects through url under this field.
3.increased the size of "string value column" in CUSTOMFIELDVALUE TABLE TO 21000.
(before increasing the size of this not able to added more than 2 links, now able to add more than two links)
4.Now, after adding more than 2 links, when tried to edit the issue fields such as title, description etc.. getting below error.
customfield_10300: "The entered text is too long. It exceeds the allowed limit of 255 characters."