JAR-FCL does not limit the number of type ratings that a pilot may hold. However, JAR-OPS does limit the number of types that a pilot may operate. Appendix 1 to JAR-OPS 1.980 states:
(b) When a flight crew member operates more than one aeroplane type or variant within one or more licence endorsement as defined by AMC FCL 1.220B (type - multi-pilot), an operator shall ensure that:
(1) The minimum flight crew complement specified in the operations Manual is the same for each type or variant to be operated;
(2) A flight crew member does not operate more than two aeroplane types or variants for which a separate licence endorsement is required; and
(3) Only aeroplanes within one licence endorsement are flown in any one flight duty period unless the operator has established
procedures to ensure adequate time for preparation.
Furthermore, JAR-FCL 1.980 itself states:
(b) When considering operations of more than one type or variant, an operator shall ensure that the differences and/or similarities of the aeroplanes concerned justify such operations, taking account of the following:
(1) The level of technology;
(2) Operational procedures;
(3) Handling characteristics.
Your hypothetical 747 pilot would, therefore, not be likely to maintain his type rating unless he was transferred to a fleet with similar handling characteristics and operational procedures. Eventually, therefore, that rating would lapse and, if ever needed again, would have to be renewed in accordance with JAR-FCL. However, a 737 pilot might also maintain a rating on, for example, an A320, if his employer operated a mixed fleet.
Before anyone jumps down my throat, this is a simplification of the overall situation and there are doubtless many scenarios that I have not addressed.