Neighbors for an LTE Cell is mostly limited by the vendors equipment capacity, still to share atleast most of the vendors currently supports:
Max No. of Intra LTE Neighbors within same eNb equal to no. of cells in eNB minus 1 (anyway all the cells under an eNB have to be nbrs) & Neighbors from other eNb (its 128) basically all together its 128 max per cell (so for enb with 3 cells no. of nbrs = 128x3=384)
Max No. of nbrs for IRAT (GSM/CDMA/3G) per eNb cell (its 50 to GSM, 50 for wcdma and 50 to cdma, again that itself seems too many)
Max no of nbrs which can be defined on S1 interface (you can define as many for S1 from 128, considering you don't overload your core, recommended is 20-25%)
Max no of nbrs which can be defined on X2 interface (you can define as many for X2 from 128, keep mostly X2 type nbr relations)
By the way, operators mostly are fond of having UE based nbrs now a days for LTE network, wherein run time nbr creations based on UE measurement is possible.