It's my understanding that sign languages combine a mix of directly signing words with fingerspelling words using a manual alphabet. What exactly is the role of fingerspelling? Is it only used to spell words that don't have a corresponding sign, or does it have a more dynamic role? How common is fingerspelling among fluent signers?
Fingerspelling is usually used for spelling out words that do not have a sign. For example, given names may often be spelled out (unless that person has a known sign).
The same goes for countries, cities, loan words, product names etc. when the signer either doesn't know the sign, or there is no sign.
Whether spelling out words has any additional function, i.e. people choosing to spell out words that everyone in the conversation knows the sign for... I'm not so sure. :)
EDIT: Apparently, fingerspelling can also be used to emphasise words. For instance, in Swedish SL one might spell the word #IDIOT rather than sign it.
Source: Basic knowledge of Swedish Sign Language
As an answer to your second question, I searched the Swedish Sign Language Corpus (http://www.ling.su.se/teckensprakskorpus), where the total number of signs is roughly around 28 000 (depending on how you define a sign, please see http://su.diva-portal.org/smash/record.jsf?pid=diva2:186542 for further reading).
Fingerspelled signs accounted for about 6.2% of the signs in the corpus.