Welcome to the forum. This topic has been covered numerous times in the past, so if you do a search you should find a lot of useful information on this topic. You didn't mention what code you are working to which does affect the answer somewhat, though it really comes down to engineering on a specific joint. I don't believe the ASME addresses minimums on this subject, nor the API other than in calculations for stress. AWS D1.1 has addressed this in Sec. 5 - Fabrication. 5.22.3.1 states for tubular connections "No two girth welds shall be located closer than one pipe diameter or 3 ft., whichever is less. There shall be no more than two girth welds in any 10 ft. interval of pipe, except as may be agreed upon the Owner and Contractor." But if your not working to this standard that won't mean anything to you. I've seen spec's for pipeline call out for one diameter but not less than 3 ft., no less than 3ft. period and no less than 8 ft. Hopefully engineering was a factor in the specifications, not my job to second guess their criteria, however once in awhile curiosity will get the best of me. I recommend doing a search here on the forum. You will find much more useful info than what I have just given. Best luck.....