Typically yes only once and they will be fertile for the remainder of their life however most breeder's usually try and mate her again after a few ooths to make sure fertility stays up, yes mantids can have many egg cases, i've heard some have had up to 12+ in a females lifespan. Females live 4-8 months after reaching adulthood depending on the mantis species and how warm you keep them and how much food your giving her, she wont simply "die" from just laying a ooth, she will lay a ooth on average every 2-3 weeks after the initial 2-3 week waiting period for the female to fully mature after that last moult to start breeding, some species will lay every week though ^^