Number of invitations at once

The UK Immigration Rules have no directive or restriction on frequency of an invitation. What is outlined is the following (added emphasis mine):

Funds, maintenance and accommodation provided by a third party

V 4.3 A visitor’s travel, maintenance and accommodation may be provided by a third party where the decision maker is satisfied that they:

(a) have a genuine professional or personal relationship with the visitor; and
(b) are not, or will not be, in breach of UK immigration laws at the time of decision or the visitor’s entry to the UK; and
(c) can and will provide support to the visitor for the intended duration of their stay.

V 4.4 The third party may be asked to give an undertaking in writing to be responsible for the applicant’s maintenance and accommodation. In this case paragraph 35 of Part 1 of these Rules applies also to Visitors. An applicant will normally be refused where, having been requested to do so, the applicant fails to provide a valid written undertaking from a third party to be responsible for their maintenance and accommodation for the period of any visit.

Your sponsor should also provide a covering letter addressed to the Entry Clearance Officer to explain the background to the invitation, for example why they are inviting you, their relationship to you, confirmation that they will provide maintenance & accommodation and have the capacity to do so.

In offering to host your friends, you would want to compose an invitation that says:

• you are willing to accommodate them during a specfic time period (dates)
• your status in the UK (e.g., citizen, legal presence)
• the location of the property (address)
• that you have the right or permission to host (e.g., ownership, tenancy agreement, letter from landlord/housing authority)
• your contact information (phone, email)