As @Dave Jones commented, problem is in function foto_segs:

import serial
import os
from picamera import PiCamera
from time import sleep

# Initialize camera global amd only once
camera = PiCamera()

def foto_segs(i):
    j=0
    for j in range(2):
        for i in range(i+8):
            camera.capture('%d.jpg' %i)
while True:
    #try:
    txt = arduino.readline()
    print txt.rstrip('\r\n')
    if (txt.rstrip('\r\n') == 'camara'):
        foto_segs(count)
        count =count+18
        print count 

As @Dave Jones commented, problem is in function foto_segs():

import serial
import os
from picamera import PiCamera
from time import sleep

# Initialize camera global amd only once
camera = PiCamera()

def foto_segs(i):
    j=0
    for j in range(2):
        for i in range(i+8):
            camera.capture('%d.jpg' %i)
while True:
    #try:
    txt = arduino.readline()
    print txt.rstrip('\r\n')
    if (txt.rstrip('\r\n') == 'camara'):
        foto_segs(count)
        count =count+18
        print count