All the following information is taken from the data sheets for these devices, on the Cisco website.
The Cisco Catalyst 3550 switches support up to 1,005 VLANs.
The Cisco Cataylst 3750 switches support up to 1,005 VLANs per stack.
Both support up to 4000 VLAN IDs, but only 1,005 VLANs configured on these (some VLAN IDs will come from other switches). Unless you are a service provider, you will never configure anything like these numbers.
Please also note that the 3550 is now end-of-life, you should look at the 3560 switches instead.