Does the number of indexes effect the modification of table?
Every time you update a record in the table, all indexes whose entries have changed because of that also need to be updated. So there more indexes you have, the longer the update will take. 

Usually that is not a concern, but if you bulk-load a lot of table, it might be a good idea to disable all indexes (and constraints) and re-enable them after the load is done (as index rebuilding is faster than continuous updates).