In LTE, GTP tunnel ID range cannot be reused for other set of IP address.
For example, if any DL data packet received at PGW for specific UE, then PGW will send that packet to UE via SGW through GTP tunnel. This GTP tunnel was selected by PGW based on TFT. if this selected tunnel ID was associated to two SGW IP addresses then to which SGW, this packet is forwarded by PGW?
32-bit value is quite a big to hold 2 power 32 TEID's
on S11 interface - TEID is for UE. Means one pair of TEID's are enough to serve UE - irrespective to how many PDN's were created.
on S5S8c interface - TEID is for PDN. Means one pair of TEID's will serve on PDN.(eg: 3 pair od TEID's will serve 3 PDN's for same UE)
In the above control plane TEID's there is no question of exhausted in case of LTE devices. This is the operator responsibility to deploy enough number of TA's/SGW's based on number of users.
on S1U/S5S8U - TEID per bearer. Means one pair of TEID's will serve on bearer(default/dedicated). Usually PGW will change(but not compulsory) per PDN/APN. So, in this case also TEID's are enough to serve.
In case of IOT devices separate diameter interface(T6a) was defined between MME and SCEF.