Are you unsure of local, RFU, league or cup regulations,?
The county leagues hand book contains all the necessary info including the following which differs from the RFU leagues. For example:-
1 max squad size = 20 – ie 5 replacements (implies 10 interchanges, as per rfu reg 13 appendix 2 para 20 )
2 must have 4 FR in squad – no mention on how to reduce this if they only have 3 or less?
3 when team starts with less than 15 the opposition may only start with one more player – ie 13 v 14, same as the merit leagues. However, unlike the merit leagues, the team with the full squad can use all non-starting players as replacements (in this case if they start with 14 they have 6 replacements) – no info is given on whether the no of interchanges is changed from 10. It could feasibly be that the match starts with 8 v 9 and the team with 9 is allowed 10 interchanges to use their 11 replacements, meaning at least one replacement doesn’t get the chance to play!
The information this has been taken from is: https://s3-eu-west-1.amazonaws.com/files.pitchero.com/counties/19/1537180590.pdf?AWSAccessKeyId=AKIAITBHWE46F2QIZWKQ&Expires=1548444332&Signature=2TGKcwkprcO8ELYwJOwf8V64TH8%3D
And can be found on the ADM leagues website: http://lancashirerfu.pitchero.com/
And on the Cheshire website: https://www.cheshirerfu.co.uk/information/downloads/