There is no limit on times but duration yes ! You can not have a long trip of more than 1 year without any type of permit ( re-entry permit).But main things where is you residence is . If you are living other side of boarder in Mexico / Canada and do a job in USA. You are called commuting resident. It affects your physical presence and naturalization.If you have no ties with US and living / working in other country ( without N470), they will consider that you have given up your residency even if you are frequently travelling.