I wanted to know how many players a club can sell during the transfer period? Are there any rules that FIFA (or UEFA) have issued regarding this matter?
FIFA has no limitations. You could basically sell your whole squad and hire a new one.
UEFA has no limits when selling players, but the Financial Fair Play (FFP) rules must be followed when buying new players. Basically they are only allowing a certain amount of expenses exceeding the club's budget.
Also UEFA has a homegrown player rule, which may as well be stricter in domestic competitions (or at least in Germany).
Article 1 of the FIFA Regulations on the Status and Transfer of Players states that international transfer rules are governed by FIFA, whereas same-association transfers are governed by the association regulations (approved by FIFA).
There's no limit in FIFA regulations on the number of players being sold (or bought) during a single transfer period. As stated above, member associations (e.g. The FA, Royal Spanish Football Federation etc.) can have their own transfer regulations (approved by FIFA), defining a limit on number of released/registered players which is, however, usually not the case.
Here's the excerpt from FIFA regulations:
Article 1 - Scope
These regulations lay down global and binding rules concerning the status of players, their eligibility to participate in organised football, and their transfer between clubs belonging to different associations.
The transfer of players between clubs belonging to the same association is governed by specific regulations issued by the association concerned in accordance with article 1 paragraph 3 below, which must be approved by FIFA.
Additionally, confederations (e.g UEFA, CONMEBOL) can have their own subset of rules regarding transfers. UEFA, famously, has Financial Fair Play Regulations, which prevent clubs spending more than they earn. Here are some relevant links:
- Is there a limit on a team's spending in one transfer window?
- Financial Fair Play explained on uefa.com
UEFA also requires minimum of eight homegrown players in 25-man squad in UEFA Competitions.