Depends entirely on numbers and the criteria of guests at the wedding.
Small, close family only and venue strict on numbers and small budget, then no.
A larger wedding with extended family and friends I think I would allow 3 friends.
Traditionally (especially pre 90s) the wedding was hosted by the brides parents and the parent's paid for everything. Sometimes the bride and grooms parents went halves, sometimes brides family paid for wedding and wedding breakfast and grooms paid for night do, sometimes brides parents paid it all. The parents invited everyone - usually their friends. My mum said she barely knew anyone outside of the family at her wedding as my granny (her MIL) invited a lot of friends. Nowadays, brides and grooms tend to pay for their own weddings and have more control over the guest list.
When I got married DH and I invited our friends and extended family and we had allowance in the numbers, and my family is much bigger than DH's so I did tell MIL she could invite a couple of friends to the wedding. My parents invited their friends to the night do as most of the whole day guests were my side of the family.
It would be nice if budget/ numbers allow to have a couple of MIL's friends there if the bride's side is larger than the groom's.
A compromise would be that MIL's friends can come to the night do when generally numbers are more relaxed.