What you need to remember with all FEX models is that they don't perform any local switching. All traffic is sent to the parent switch that provides central forwarding and policy enforcement. See below:
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/layer2/502_n1_1m/b_Cisco_n5k_layer2_config_gd_rel_502_N1_1/Cisco_n5k_layer2_config_gd_rel_502_N1_1_chapter16.html
Is one uplink enough? That depends on how much traffic is being generated by the end devices connected to the FEX. This gives you no redundnancy if this single link fails. Furthermore as stated above since all processing is done on the parent switch, the more uplinks the better.
The 5548P supports up to 24 x FEXes or 16 if they are running in Layer 3 mode:
http://www.cisco.com/c/en/us/products/collateral/switches/nexus-2000-series-fabric-extenders/data_sheet_c78-507093.html
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