arcmap field calculator increment number

rec=0
def autoIncrement():
 global rec
 pStart = 1
 pInterval = 1
 if (rec == 0):
  rec = pStart
 else:
  rec += pInterval
 return rec

Note:  this filed is calculated filed, subject to change when you join.
So you should create text string field ID, by copy this field convert to string below.

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create string ID:

vbsript :   Cstr()  convert int to string

cstr( [id] )