you don't need 2 DNS servers, if you only have 1 now then using just 1 is the same as you are right now
however 2 DNS servers is always better as if one fails then everything goes down, and to be honest a DNS server is such a simple thing that can be put on to any spare computer laying around (even for large ISPs the resource usage is negligible) so for an ISP to only have 1 is rather silly of them considering the small effort required for a backup
...but yes, if you just use the 1 then it will be just the same as you currently are if you only have 1 set, although it wouldn't hurt for you to get a backup DNS Server from your ISP (or if they don't have a second, suggest they set one up, benefiting all customers with very little effort)